Symmetric multiprocessing

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Symmetric_multiprocessing 

 

Diagram of a symmetric multiprocessing system

Symmetric multiprocessing (SMP) involves a multiprocessor computer hardware and software architecture where two or more identical processors are connected to a single, shared main memory, have full access to all input and output devices, and are controlled by a single operating system instance that treats all processors equally, reserving none for special purposes. Most multiprocessor systems today use an SMP architecture. In the case of multi-core processors, the SMP architecture applies to the cores, treating them as separate processors.

Professor John D. Kubiatowicz considers traditionally SMP systems to contain processors without caches. Culler and Pal-Singh in their 1998 book "Parallel Computer Architecture: A Hardware/Software Approach" mention: "The term SMP is widely used but causes a bit of confusion. [...] The more precise description of what is intended by SMP is a shared memory multiprocessor where the cost of accessing a memory location is the same for all processors; that is, it has uniform access costs when the access actually is to memory. If the location is cached, the access will be faster, but cache access times and memory access times are the same on all processors."

SMP systems are tightly coupled multiprocessor systems with a pool of homogeneous processors running independently of each other. Each processor, executing different programs and working on different sets of data, has the capability of sharing common resources (memory, I/O device, interrupt system and so on) that are connected using a system bus or a crossbar. 

Design

SMP systems have centralized shared memory called main memory (MM) operating under a single operating system with two or more homogeneous processors. Usually each processor has an associated private high-speed memory known as cache memory (or cache) to speed up the main memory data access and to reduce the system bus traffic.

Processors may be interconnected using buses, crossbar switches or on-chip mesh networks. The bottleneck in the scalability of SMP using buses or crossbar switches is the bandwidth and power consumption of the interconnect among the various processors, the memory, and the disk arrays. Mesh architectures avoid these bottlenecks, and provide nearly linear scalability to much higher processor counts at the sacrifice of programmability:

Serious programming challenges remain with this kind of architecture because it requires two distinct modes of programming; one for the CPUs themselves and one for the interconnect between the CPUs. A single programming language would have to be able to not only partition the workload, but also comprehend the memory locality, which is severe in a mesh-based architecture.

SMP systems allow any processor to work on any task no matter where the data for that task is located in memory, provided that each task in the system is not in execution on two or more processors at the same time. With proper operating system support, SMP systems can easily move tasks between processors to balance the workload efficiently.

History

The earliest production system with multiple identical processors was the Burroughs B5000, which was functional around 1961. However at run-time this was asymmetric, with one processor restricted to application programs while the other processor mainly handled the operating system and hardware interrupts. The Burroughs D825 first implemented SMP in 1962.

IBM offered dual-processor computer systems based on its System/360 Model 65 and the closely related Model 67 and 67-2. The operating systems that ran on these machines were OS/360 M65MP and TSS/360. Other software developed at universities, notably the Michigan Terminal System (MTS), used both CPUs. Both processors could access data channels and initiate I/O. In OS/360 M65MP, peripherals could generally be attached to either processor since the operating system kernel ran on both processors (though with a "big lock" around the I/O handler). The MTS supervisor (UMMPS) has the ability to run on both CPUs of the IBM System/360 model 67-2. Supervisor locks were small and used to protect individual common data structures that might be accessed simultaneously from either CPU.

Other mainframes that supported SMP included the UNIVAC 1108 II, released in 1965, which supported up to three CPUs, and the GE-635 and GE-645, although GECOS on multiprocessor GE-635 systems ran in a master-slave asymmetric fashion, unlike Multics on multiprocessor GE-645 systems, which ran in a symmetric fashion.

Starting with its version 7.0 (1972), Digital Equipment Corporation's operating system TOPS-10 implemented the SMP feature, the earliest system running SMP was the DECSystem 1077 dual KI10 processor system. Later KL10 system could aggregate up to 8 CPUs in a SMP manner. In contrast, DECs first multi-processor VAX system, the VAX-11/782, was asymmetric, but later VAX multiprocessor systems were SMP.

Early commercial Unix SMP implementations included the Sequent Computer Systems Balance 8000 (released in 1984) and Balance 21000 (released in 1986). Both models were based on 10 MHz National Semiconductor NS32032 processors, each with a small write-through cache connected to a common memory to form a shared memory system. Another early commercial Unix SMP implementation was the NUMA based Honeywell Information Systems Italy XPS-100 designed by Dan Gielan of VAST Corporation in 1985. Its design supported up to 14 processors, but due to electrical limitations, the largest marketed version was a dual processor system. The operating system was derived and ported by VAST Corporation from AT&T 3B20 Unix SysVr3 code used internally within AT&T. 

Earlier non-commercial multiprocessing UNIX ports existed, including a port named MUNIX created at the Naval Postgraduate School by 1975.

Uses

Time-sharing and server systems can often use SMP without changes to applications, as they may have multiple processes running in parallel, and a system with more than one process running can run different processes on different processors. 

On personal computers, SMP is less useful for applications that have not been modified. If the system rarely runs more than one process at a time, SMP is useful only for applications that have been modified for multithreaded (multitasked) processing. Custom-programmed software can be written or modified to use multiple threads, so that it can make use of multiple processors. 

Multithreaded programs can also be used in time-sharing and server systems that support multithreading, allowing them to make more use of multiple processors.

Advantages/Disadvantages

In current SMP systems, all of the processors are tightly coupled inside the same box with a bus or switch; on earlier SMP systems, a single CPU took an entire cabinet. Some of the components that are shared are global memory, disks, and I/O devices. Only one copy of an OS runs on all the processors, and the OS must be designed to take advantage of this architecture. Some of the basic advantages involves cost-effective ways to increase throughput. To solve different problems and tasks, SMP applies multiple processors to that one problem, known as parallel programming.

However, there are a few limits on the scalability of SMP due to cache coherence and shared objects. 

Programming

Uniprocessor and SMP systems require different programming methods to achieve maximum performance. Programs running on SMP systems may experience an increase in performance even when they have been written for uniprocessor systems. This is because hardware interrupts usually suspends program execution while the kernel that handles them can execute on an idle processor instead. The effect in most applications (e.g. games) is not so much a performance increase as the appearance that the program is running much more smoothly. Some applications, particularly building software and some distributed computing projects, run faster by a factor of (nearly) the number of additional processors. (Compilers by themselves are single threaded, but, when building a software project with multiple compilation units, if each compilation unit is handled independently, this creates an embarrassingly parallel situation across the entire multi-compilation-unit project, allowing near linear scaling of compilation time. Distributed computing projects are inherently parallel by design.)

Systems programmers must build support for SMP into the operating system, otherwise, the additional processors remain idle and the system functions as a uniprocessor system.

SMP systems can also lead to more complexity regarding instruction sets. A homogeneous processor system typically requires extra registers for "special instructions" such as SIMD (MMX, SSE, etc.), while a heterogeneous system can implement different types of hardware for different instructions/uses. 

Performance

When more than one program executes at the same time, an SMP system has considerably better performance than a uni-processor, because different programs can run on different CPUs simultaneously. Similarly, Asymmetric multiprocessing (AMP) usually allows only one processor to run a program or task at a time. For example, AMP can be used in assigning specific tasks to CPU based to priority and importance of task completion. AMP was created well before SMP in terms of handling multiple CPUs, which explains the lack of performance based on the example provided.

In cases where an SMP environment processes many jobs, administrators often experience a loss of hardware efficiency. Software programs have been developed to schedule jobs and other functions of the computer so that the processor utilization reaches its maximum potential. Good software packages can achieve this maximum potential by scheduling each CPU separately, as well as being able to integrate multiple SMP machines and clusters. 

Access to RAM is serialized; this and cache coherency issues causes performance to lag slightly behind the number of additional processors in the system. 

Alternatives

Diagram of a typical SMP system. Three processors are connected to the same memory module through a system bus or crossbar switch

SMP uses a single shared system bus that represents one of the earliest styles of multiprocessor machine architectures, typically used for building smaller computers with up to 8 processors.

Larger computer systems might use newer architectures such as NUMA (Non-Uniform Memory Access), which dedicates different memory banks to different processors. In a NUMA architecture, processors may access local memory quickly and remote memory more slowly. This can dramatically improve memory throughput as long as the data are localized to specific processes (and thus processors). On the downside, NUMA makes the cost of moving data from one processor to another, as in workload balancing, more expensive. The benefits of NUMA are limited to particular workloads, notably on servers where the data are often associated strongly with certain tasks or users.

Finally, there is computer clustered multiprocessing (such as Beowulf), in which not all memory is available to all processors. Clustering techniques are used fairly extensively to build very large supercomputers. 

Variable SMP

Variable Symmetric Multiprocessing (vSMP) is a specific mobile use case technology initiated by NVIDIA. This technology includes an extra fifth core in a quad-core device, called the Companion core, built specifically for executing tasks at a lower frequency during mobile active standby mode, video playback, and music playback. 

Project Kal-El (Tegra 3), patented by NVIDIA, was the first SoC (System on Chip) to implement this new vSMP technology. This technology not only reduces mobile power consumption during active standby state, but also maximizes quad core performance during active usage for intensive mobile applications. Overall this technology addresses the need for increase in battery life performance during active and standby usage by reducing the power consumption in mobile processors.

Unlike current SMP architectures, the vSMP Companion core is OS transparent meaning that the operating system and the running applications are totally unaware of this extra core but are still able to take advantage of it. Some of the advantages of the vSMP architecture includes cache coherency, OS efficiency, and power optimization. The advantages for this architecture are explained below:

• Cache Coherency: There are no consequences for synchronizing caches between cores running at different frequencies since vSMP does not allow the Companion core and the main cores to run simultaneously.
• OS Efficiency: It is inefficient when multiple CPU cores are run at different asynchronous frequencies because this could lead to possible scheduling issues. With vSMP, the active CPU cores will run at similar frequencies to optimize OS scheduling.
• Power Optimization: In asynchronous clocking based architecture, each core is on a different power plane to handle voltage adjustments for different operating frequencies. The result of this could impact performance. vSMP technology is able to dynamically enable and disable certain cores for active and standby usage, reducing overall power consumption.

These advantages lead the vSMP architecture to considerably benefit over other architectures using asynchronous clocking technologies.

Asymmetric multiprocessing

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Asymmetry

An asymmetric multiprocessing (AMP) system is a multiprocessor computer system where not all of the multiple interconnected central processing units (CPUs) are treated equally. For example, a system might allow (either at the hardware or operating system level) only one CPU to execute operating system code or might allow only one CPU to perform I/O operations. Other AMP systems might allow any CPU to execute operating system code and perform I/O operations, so that they were symmetric with regard to processor roles, but attached some or all peripherals to particular CPUs, so that they were asymmetric with respect to the peripheral attachment.

Asymmetric multiprocessing was the only method for handling multiple CPUs before symmetric multiprocessing (SMP) was available. It has also been used to provide less expensive options on systems where SMP was available. Additionally, AMP is used in applications that are dedicated, such as embedded systems, when individual processors can be dedicated to specific tasks at design time.

Asymmetric multiprocessing

 

Background and history

For the room-size computers of the 1960s and 1970s, a cost-effective way to increase compute power was to add a second CPU. Since these computers were already close to the fastest available (near the peak of the price:performance ratio), two standard-speed CPUs were much less expensive than a CPU that ran twice as fast. Also, adding a second CPU was less expensive than a second complete computer, which would need its own peripherals, thus requiring much more floor space and an increased operations staff. 

Notable early AMP offerings by computer manufacturers were the Burroughs B5000, the DECsystem-1055, and the IBM System/360 model 65MP. There were also dual-CPU machines built at universities.

The problem with adding a second CPU to a computer system was that the operating system had been developed for single-CPU systems, and extending it to handle multiple CPUs efficiently and reliably took a long time. To fill this gap, operating systems intended for single CPUs were initially extended to provide minimal support for a second CPU. In this minimal support, the operating system ran on the “boot” processor, with the other only allowed to run user programs. In the case of the Burroughs B5000, the second processor's hardware was not capable of running "control state" code.

Other systems allowed the operating system to run on all processors, but either attached all the peripherals to one processor or attached particular peripherals to particular processors. 

Burroughs B5000 and B5500

An option on the Burroughs B5000 was “Processor B”. This second processor, unlike “Processor A” had no connection to the peripherals, though the two processors shared main memory, and Processor B could not run in Control State. The operating system ran only on Processor A. When there was a user job to be executed, it might be run on Processor B, but when that job tried to access the operating system the processor halted and signaled Processor A. The requested operating system service was then run on Processor A. 

On the B5500, either Processor A or Processor B could be designated as Processor 1 by a switch on the engineer's panel, with the other processor being Processor 2; both processors shared main memory and had hardware access to the I/O processors hence the peripherals, but only Processor 1 could respond to peripheral interrupts. When a job on Processor 2 required an operating system service it would be rescheduled on Processor 1, which was responsible for both initiating I/O processor activity and responding to interrupts indicating completion. In practice, this meant that while user jobs could run on either Processor 1 or Processor 2 and could access intrinsic library routines that didn't require kernel support, the operating system would schedule them on the latter whenever possible.

CDC 6500 and 6700

Control Data Corporation offered two configurations of its CDC 6000 series that featured two central processors. The CDC 6500 was a CDC 6400 with two central processors. The CDC 6700 was a CDC 6600 with the CDC 6400 central processor added to it.

These systems were organized quite differently from the other multiprocessors in this article. The operating system ran on the peripheral processors, while the user's application ran on the CPUs. Thus, the terms ASMP and SMP do not properly apply to these multiprocessors. 

DECsystem-10

Digital Equipment Corporation (DEC) offered a dual-processor version of its DECsystem-1050 which used two KA10 processors; all peripherals were attached to one processor, the primary processor, and the primary processor ran the operating system code. This offering was extended to the KL-10 and KS-10 processors in the PDP-10 line; in those systems, the boot CPU is designated the "policy CPU", which runs the command interpreter, swaps jobs in and out of memory, and performs a few other functions; other operating system functions, and I/O, can be performed by any of the processors, and if the policy processor fails, another processor takes over as the policy processor.

PDP-11/74

Digital Equipment Corporation developed, but never released, a multiprocessor PDP-11, the PDP-11/74, running a multiprocessor version of RSX-11M. In that system, either processor could run operating system code, and could perform I/O, but not all peripherals were accessible to all processors; most peripherals were attached to one or the other of the CPUs, so that a processor to which a peripheral wasn't attached would, when it needed to perform an I/O operation on that peripheral, request the processor to which the peripheral was attached to perform the operation.

VAX-11/782

DEC's first multi-processor VAX system, the VAX-11/782, was an asymmetric dual-processor system; only the first processor had access to the I/O devices.

IBM System/370 model 168

Two options were available for the IBM System/370 Model 168 for attaching a second processor. One was the IBM 3062 Attached Processing Unit, in which the second processor had no access to the channels, and was therefore similar to the B5000's Processor B or the second processor on a VAX-11/782. The other option offered a complete second CPU, and was thus more like the System/360 model 65MP.

Asymmetry

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Asymmetry

Asymmetrical fire barrier, required by ASTM E119 to be fire tested on both sides. The lowest result achieved equates to the overall fire-resistance rating of the system in order to ensure that the barrier works equally well from both sides.

 

Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in precise terms or in more aesthetic terms. The absence of or violation of symmetry that are either expected or desired can have important consequences for a system.

In organisms

Due to how cells divide in organisms, asymmetry in organisms is fairly usual in at least one dimension, with biological symmetry also being common in at least one dimension. 

Louis Pasteur proposed that biological molecules are asymmetric because the cosmic [i.e. physical] forces that preside over their formation are themselves asymmetric. While at his time, and even now, the symmetry of physical processes are highlighted, it is known that there are fundamental physical asymmetries, starting with time. 

Asymmetry in biology

Asymmetry is an important and widespread trait, having evolved numerous times in many organisms and at many levels of organisation (ranging from individual cells, through organs, to entire body-shapes). Benefits of asymmetry sometimes have to do with improved spatial arrangements, such as the left human lung being smaller, and having one fewer lobes than the right lung to make room for the asymmetrical heart. In other examples, division of function between the right and left half may have been beneficial and has driven the asymmetry to become stronger. Such an explanation is usually given for mammal hand or paw preference (Handedness), an asymmetry in skill development in mammals. Training the neural pathways in a skill with one hand (or paw) may take less effort than doing the same with both hands.

Nature also provides several examples of handedness in traits that are usually symmetric. The following are examples of animals with obvious left-right asymmetries: 

Fiddler crab, Uca pugnax

• Most snails, because of torsion during development, show remarkable asymmetry in the shell and in the internal organs.
• Fiddler crabs have one big claw and one small claw.
• The narwhal's tusk is a left incisor which can grow up to 10 feet in length and forms a left-handed helix.
• Flatfish have evolved to swim with one side upward, and as a result have both eyes on one side of their heads.
• Several species of owls exhibit asymmetries in the size and positioning of their ears, which is thought to help locate prey.
• Many animals (ranging from insects to mammals) have asymmetric male genitalia. The evolutionary cause behind this is, in most cases, still a mystery.

As an indicator of unfitness

• Certain disturbances during the development of the organism, resulting in birth defects.
• Injuries after cell division that cannot be biologically repaired, such as a lost limb from an accident.

Since birth defects and injuries are likely to indicate poor health of the organism, defects resulting in asymmetry often put an animal at a disadvantage when it comes to finding a mate. In particular, a degree of facial symmetry is associated with physical attractiveness, but complete symmetry is both impossible and probably unattractive.

In structures

Pre-modern architectural styles tended to place an emphasis on symmetry, except where extreme site conditions or historical developments lead away from this classical ideal. To the contrary, modernist and postmodern architects became much more free to use asymmetry as a design element.

While most bridges employ a symmetrical form due to intrinsic simplicities of design, analysis and fabrication and economical use of materials, a number of modern bridges have deliberately departed from this, either in response to site-specific considerations or to create a dramatic design statement.

Some asymmetrical structures

• 

  Eastern span replacement of the San Francisco – Oakland Bay Bridge
  
  
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  Puente de la Mujer
  
  
• 

  Auditorio de Tenerife
  
• 

  Blohm & Voss BV 141 aircraft
  
  
• 

  A proa, a form of outrigger canoe
  
  

In fire protection

In fire-resistance rated wall assemblies, used in passive fire protection, including, but not limited to, high-voltage transformer fire barriers, asymmetry is a crucial aspect of design. When designing a facility, it is not always certain, that in the event of fire, which side a fire may come from. Therefore, many building codes and fire test standards outline, that a symmetrical assembly, need only be tested from one side, because both sides are the same. However, as soon as an assembly is asymmetrical, both sides must be tested and the test report is required to state the results for each side. In practical use, the lowest result achieved is the one that turns up in certification listings. Neither the test sponsor, nor the laboratory can go by an opinion or deduction as to which side was in more peril as a result of contemplated testing and then test only one side. Both must be tested in order to be compliant with test standards and building codes.

 

In mathematics

There are no a and b such that a < b and b < a. This form of asymmetry is an asymmetrical relation.

In chemistry

Certain molecules are chiral; that is, they cannot be superposed upon their mirror image. Chemically identical molecules with different chirality are called enantiomers; this difference in orientation can lead to different properties in the way they react with biological systems. 

In physics

Asymmetry arises in physics in a number of different realms. 

Thermodynamics

The original non-statistical formulation of thermodynamics was asymmetrical in time: it claimed that the entropy in a closed system can only increase with time. This was derived from the Second Law (any of the two, Clausius' or Lord Kelvin's statement can be used since they are equivalent) and using the Clausius' Theorem (see Kerson Huang ISBN 978-0471815181). The later theory of statistical mechanics, however, is symmetric in time. Although it states that a system significantly below maximum entropy is very likely to evolve towards higher entropy, it also states that such a system is very likely to have evolved from higher entropy. 

Particle physics

Symmetry is one of the most powerful tools in particle physics, because it has become evident that practically all laws of nature originate in symmetries. Violations of symmetry therefore present theoretical and experimental puzzles that lead to a deeper understanding of nature. Asymmetries in experimental measurements also provide powerful handles that are often relatively free from background or systematic uncertainties.

Parity violation

Until the 1950s, it was believed that fundamental physics was left-right symmetric; i.e., that interactions were invariant under parity. Although parity is conserved in electromagnetism, strong interactions and gravity, it turns out to be violated in weak interactions. The Standard Model incorporates parity violation by expressing the weak interaction as a chiral gauge interaction. Only the left-handed components of particles and right-handed components of antiparticles participate in weak interactions in the Standard Model. A consequence of parity violation in particle physics is that neutrinos have only been observed as left-handed particles (and antineutrinos as right-handed particles). 

In 1956-1957 Chien-Shiung Wu, E. Ambler, R. W. Hayward, D. D. Hoppes, and R. P. Hudson found a clear violation of parity conservation in the beta decay of cobalt-60. Simultaneously, R. L. Garwin, Leon Lederman, and R. Weinrich modified an existing cyclotron experiment and immediately verified parity violation.

CP violation

After the discovery of the violation of parity in 1956-57, it was believed that the combined symmetry of parity (P) and simultaneous charge conjugation (C), called CP, was preserved. For example, CP transforms a left-handed neutrino into a right-handed antineutrino. In 1964, however, James Cronin and Val Fitch provided clear evidence that CP symmetry was also violated in an experiment with neutral kaons. 

CP violation is one of the necessary conditions for the generation of a baryon asymmetry in the universe.

Combining the CP symmetry with simultaneous time reversal (T) produces a combined symmetry called CPT symmetry. CPT symmetry must be preserved in any Lorentz invariant local quantum field theory with a Hermitian Hamiltonian. As of 2006, no violations of CPT symmetry have been observed.

Baryon asymmetry of the universe

The baryons (i.e., the protons and neutrons and the atoms that they comprise) observed so far in the universe are overwhelmingly matter as opposed to anti-matter. This asymmetry is called the baryon asymmetry of the universe.

Isospin violation

Isospin is the symmetry transformation of the weak interactions. The concept was first introduced by Werner Heisenberg in nuclear physics based on the observations that the masses of the neutron and the proton are almost identical and that the strength of the strong interaction between any pair of nucleons is the same, independent of whether they are protons or neutrons. This symmetry arises at a more fundamental level as a symmetry between up-type and down-type quarks. Isospin symmetry in the strong interactions can be considered as a subset of a larger flavor symmetry group, in which the strong interactions are invariant under interchange of different types of quarks. Including the strange quark in this scheme gives rise to the Eightfold Way scheme for classifying mesons and baryons.

Isospin is violated by the fact that the masses of the up and down quarks are different, as well as by their different electric charges. Because this violation is only a small effect in most processes that involve the strong interactions, isospin symmetry remains a useful calculational tool, and its violation introduces corrections to the isospin-symmetric results. 

In collider experiments

Because the weak interactions violate parity, collider processes that can involve the weak interactions typically exhibit asymmetries in the distributions of the final-state particles. These asymmetries are typically sensitive to the difference in the interaction between particles and antiparticles, or between left-handed and right-handed particles. They can thus be used as a sensitive measurement of differences in interaction strength and/or to distinguish a small asymmetric signal from a large but symmetric background.

• A forward-backward asymmetry is defined as A_FB=(N_F-N_B)/(N_F+N_B), where N_F is the number of events in which some particular final-state particle is moving "forward" with respect to some chosen direction (e.g., a final-state electron moving in the same direction as the initial-state electron beam in electron-positron collisions), while N_B is the number of events with the final-state particle moving "backward". Forward-backward asymmetries were used by the LEP experiments to measure the difference in the interaction strength of the Z boson between left-handed and right-handed fermions, which provides a precision measurement of the weak mixing angle.
• A left-right asymmetry is defined as A_LR=(N_L-N_R)/(N_L+N_R), where N_L is the number of events in which some initial- or final-state particle is left-polarized, while N_R is the corresponding number of right-polarized events. Left-right asymmetries in Z boson production and decay were measured at the Stanford Linear Collider using the event rates obtained with left-polarized versus right-polarized initial electron beams. Left-right asymmetries can also be defined as asymmetries in the polarization of final-state particles whose polarizations can be measured; e.g., tau leptons.
• A charge asymmetry or particle-antiparticle asymmetry is defined in a similar way. This type of asymmetry has been used to constrain the parton distribution functions of protons at the Tevatron from events in which a produced W boson decays to a charged lepton. The asymmetry between positively and negatively charged leptons as a function of the direction of the W boson relative to the proton beam provides information on the relative distributions of up and down quarks in the proton. Particle-antiparticle asymmetries are also used to extract measurements of CP violation from B meson and anti-B meson production at the BaBar and Belle experiments.

Lexical

Asymmetry is also relevant to grammar and linguistics, especially in the contexts of lexical analysis and transformational grammar.

Enumeration example: In English, there are grammatical rules for specifying coordinate items in an enumeration or series. Similar rules exist for programming languages and mathematical notation. These rules vary, and some require lexical asymmetry to be considered grammatically correct.

For example, in standard written English: 

   We sell domesticated cats, dogs, and goldfish.        ### in-line asymmetric and grammatical
   We sell domesticated animals (cats, dogs, goldfish).  ### in-line symmetric and grammatical
   We sell domesticated animals (cats, dogs, goldfish,). ### in-line symmetric and ungrammatical
   We sell domesticated animals:                         ### outline symmetric and grammatical
     - cats
     - dogs
     - goldfish

Chirality

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Chirality 

 

Two enantiomers of a generic amino acid that is chiral

Chirality /kaɪˈrælɪtiː/ is a property of asymmetry important in several branches of science. The word chirality is derived from the Greek χειρ (kheir), "hand," a familiar chiral object.

An object or a system is chiral if it is distinguishable from its mirror image; that is, it cannot be superposed onto it. Conversely, a mirror image of an achiral object, such as a sphere, cannot be distinguished from the object. A chiral object and its mirror image are called enantiomorphs (Greek, "opposite forms") or, when referring to molecules, enantiomers. A non-chiral object is called achiral (sometimes also amphichiral) and can be superposed on its mirror image.

The term was first used by Lord Kelvin in 1893 in the second Robert Boyle Lecture at the Oxford University Junior Scientific Club which was published in 1894:

I call any geometrical figure, or group of points, 'chiral', and say that it has chirality if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself.

Human hands are perhaps the most universally recognized example of chirality. The left hand is a non-superimposable mirror image of the right hand; no matter how the two hands are oriented, it is impossible for all the major features of both hands to coincide across all axes. This difference in symmetry becomes obvious if someone attempts to shake the right hand of a person using their left hand, or if a left-handed glove is placed on a right hand. In mathematics, chirality is the property of a figure that is not identical to its mirror image.

Mathematics

An achiral 3D object without central symmetry or a plane of symmetry

 

A table of all prime knots with seven crossings or fewer (not including mirror images).

In mathematics, a figure is chiral (and said to have chirality) if it cannot be mapped to its mirror image by rotations and translations alone. For example, a right shoe is different from a left shoe, and clockwise is different from anticlockwise.

A chiral object and its mirror image are said to be enantiomorphs. The word enantiomorph stems from the Greek ἐναντίος (enantios) 'opposite' + μορφή (morphe) 'form'. A non-chiral figure is called achiral or amphichiral.

The helix (and by extension a spun string, a screw, a propeller, etc.) and Möbius strip are chiral two-dimensional objects in three-dimensional ambient space. The J, L, S and Z-shaped tetrominoes of the popular video game Tetris also exhibit chirality, but only in a two-dimensional space.

Many other familiar objects exhibit the same chiral symmetry of the human body, such as gloves, glasses (where two lenses differ in prescription), and shoes. A similar notion of chirality is considered in knot theory, as explained below. 

Some chiral three-dimensional objects, such as the helix, can be assigned a right or left handedness, according to the right-hand rule. 

Geometry

In geometry a figure is achiral if and only if its symmetry group contains at least one orientation-reversing isometry. In two dimensions, every figure that possesses an axis of symmetry is achiral, and it can be shown that every bounded achiral figure must have an axis of symmetry. In three dimensions, every figure that possesses a plane of symmetry or a center of symmetry is achiral. There are, however, achiral figures lacking both plane and center of symmetry. In terms of point groups, all chiral figures lack an improper axis of rotation (S_n). This means that they cannot contain a center of inversion (i) or a mirror plane (σ). Only figures with a point group designation of C₁, C_n, D_n, T, O, or I can be chiral. 

Knot theory

A knot is called achiral if it can be continuously deformed into its mirror image, otherwise it is called chiral. For example, the unknot and the figure-eight knot are achiral, whereas the trefoil knot is chiral. 

Physics

Animation of left-handed (anticlockwise) circularly polarized light, as defined from the direction of the source in agreement with physics and astronomy conventions.

In physics, chirality may be found in the spin of a particle, where the handedness of the object is determined by the direction in which the particle spins. Not to be confused with helicity, which is the projection of the spin along the linear momentum of a subatomic particle, chirality is a purely quantum mechanical phenomenon like spin. Although both can have left-handed or right-handed properties, only in the massless case do they have a simple relation. In particular for a massless particle the helicity is the same as the chirality while for an antiparticle they have opposite sign. 

The handedness in both chirality and helicity relate to the rotation of a particle while it proceeds in linear motion with reference to the human hands. The thumb of the hand points towards the direction of linear motion whilst the fingers curl into the palm, representing the direction of rotation of the particle (i.e. clockwise and counterclockwise). Depending on the linear and rotational motion, the particle can either be defined by left-handedness or right-handedness. A symmetry transformation between the two is called parity. Invariance under parity by a Dirac fermion is called chiral symmetry. 

Electromagnetism

Electromagnetic wave propagation as handedness is wave polarization and described in terms of helicity (occurs as a helix). Polarization of an electromagnetic wave is the property that describes the orientation, i.e., the time-varying, direction (vector), and amplitude of the electric field vector. 

Chiral mirrors are a class of metamaterials that reflect circularly polarized light of a certain helicity in a handedness-preserving manner, while absorbing circular polarization of the opposite handedness. However, most absorbing chiral mirrors operate only in a narrow frequency band, as limited by the causality principle. Employing a different design methodology that allows undesired waves to pass through instead of absorbing the undesired waveform, chiral mirrors are able to show good performance in broadband.

Chemistry

(S)-Alanine (left) and (R)-alanine (right) in zwitterionic form at neutral pH

A chiral molecule is a type of molecule that has a non-superposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom.

The term "chiral" in general is used to describe the object that is non-superposable on its mirror image.

In chemistry, chirality usually refers to molecules. Two mirror images of a chiral molecule are called enantiomers or optical isomers. Pairs of enantiomers are often designated as "right-", "left-handed" or, if they have no bias, "achiral". As polarized light passes through a chiral molecule, the plane of polarization, when viewed along the axis toward the source, will be rotated clockwise (to the right) or anticlockwise (to the left). A right handed rotation is dextrorotary (d); that to the left is levorotary (l). The d- and l-isomers are the same compound but are called enantiomers. An equimolar mixture of the two optical isomers will produce no net rotation of polarized light as it passes through. Left handed molecules have l- prefixed to their names; d- is prefixed to right handed molecules. 

Molecular chirality is of interest because of its application to stereochemistry in inorganic chemistry, organic chemistry, physical chemistry, biochemistry, and supramolecular chemistry. 

More recent developments in chiral chemistry include the development of chiral inorganic nanoparticles that may have the similar tetrahedral geometry as chiral centers associated with sp3 carbon atoms traditionally associated with chiral compounds, but at larger scale. Helical and other symmetries of chiral nanomaterials were also obtained.

Biology

All of the known life-forms show specific chiral properties in chemical structures as well as macroscopic anatomy, development and behavior. In any specific organism or evolutionarily related set thereof, individual compounds, organs, or behavior are found in the same single enantiomorphic form. Deviation (having the opposite form) could be found in a small number of chemical compounds, or certain organ or behavior but that variation strictly depends upon the genetic make up of the organism. From chemical level (molecular scale), biological systems show extreme stereospecificity in synthesis, uptake, sensing, metabolic processing. A living system usually deals with two enantiomers of the same compound in drastically different ways.

In biology, homochirality is a common property of amino acids and carbohydrates. The chiral protein-making amino acids, which are translated through the ribosome from genetic coding, occur in the L form. However, D-amino acids are also found in nature. The monosaccharides (carbohydrate-units) are commonly found in D-configuration. DNA double helix is chiral (as any kind of helix is chiral), and B-form of DNA shows a right-handed turn.

R-(+)-Limonene found in orange

 

S-(–)-Limonene found in lemon

 

Sometimes, when two enantiomers of a compound found in organisms, they significantly differ in their taste, smell and other biological actions. For example, (+)-limonene found in orange (causing its smell), and (–)-limonene found in lemons (causing its smell), show different smells due to different biochemical interactions at human nose. (+)-Carvone is responsible for the smell of caraway seed oil, whereas (–)-carvone is responsible for smell of spearmint oil.

(S)-(+)-Carvone occurs in caraway seed oil, and (R)-(-)-carvone occurs in spearmint

 

Dextropropoxyphene or Darvon, a painkiller

 

Levopropoxyphene or Novrad, an anticough agent

Also, for artificial compounds, including medicines, in case of chiral drugs, the two enantiomers sometimes show remarkable difference in effect of their biological actions. Darvon (dextropropoxyphene) is a painkiller, whereas its enantiomer, Novrad (levopropoxyphene) is an anti-cough agent. In case of penicillamine, the (S-isomer used in treatment of primary chronic arthritis, whereas the (R)-isomer has no therapeutic effect as well as being highly toxic. In some cases the less therapeutically active enantiomer can cause side effects. For example, (S-naproxen is an analgesic but the (R-isomer cause renal problems. The naturally occurring plant form of alpha-tocopherol (vitamin E) is RRR-α-tocopherol whereas the synthetic form (all-racemic vitamin E, or dl-tocopherol) is equal parts of the stereoisomers RRR, RRS, RSS, SSS, RSR, SRS, SRR and SSR with progressively decreasing biological equivalency, so that 1.36 mg of dl-tocopherol is considered equivalent to 1.0 mg of d-tocopherol.

A natural left-handed helix, made by a certain climber plant's tendril.

Macroscopic examples of chirality are found in the plant kingdom, the animal kingdom and all other groups of organism. A simple example is the coiling direction of any climber plant, which can grow to form either a left- or right-handed helix.

Shells of two different species of sea snail: on the left is the normally sinistral (left-handed) shell of Neptunea angulata, on the right is the normally dextral (right-handed) shell of Neptunea despecta

 

In anatomy, chirality is found in the imperfect mirror image symmetry of many kinds of animal bodies. Organisms such as gastropods exhibit chirality in their coiled shells, resulting in an asymmetrical appearance. Over 90% of gastropod species  have dextral (right-handed) shells in their coiling, but a small minority of species and genera are virtually always sinistral (left-handed). A very few species (for example Amphidromus perversus) show an equal mixture of dextral and sinistral individuals. 

In humans, chirality (also referred to as handedness or laterality) is an attribute of humans defined by their unequal distribution of fine motor skill between the left and right hands. An individual who is more dexterous with the right hand is called right-handed, and one who is more skilled with the left is said to be left-handed. Chirality is also seen in the study of facial asymmetry. 

In flatfish, the Summer flounder or fluke are left-eyed, while halibut are right-eyed.

Popular Culture

Chirality, chiral theory and even "knots" of a sort feature very heavily in the 2019 video game Death Stranding, by Hideo Kojima and Kojima Productions.

Tocotrienol

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Tocotrienol

 

General chemical structure of tocotrienols. alpha(α)-Tocotrienol: R1 = Me, R2 = Me, R3 = Me; beta(β)-Tocotrienol: R1 = Me, R2 = H, R3= Me; gamma(γ)-Tocotrienol: R1 = H, R2 = Me, R3= Me; delta(δ)-Tocotrienol: R1 = H, R2 = H, R3= Me

The vitamin E family comprise four tocotrienols (alpha, beta, gamma, delta) and four tocopherols (alpha, beta, gamma, delta). The critical chemical structural difference between tocotrienols and tocopherols is that tocotrienols have unsaturated isoprenoid side chains with three carbon-carbon double bonds versus saturated side chains for tocopherols (see Figure).

Tocotrienols are compounds naturally occurring at higher levels in some vegetable oils, including palm oil, rice bran oil, wheat germ, barley, saw palmetto, annatto, and certain other types of seeds, nuts and grains, and the oils derived from them.

Chemically, different analogues of vitamin E all show some activity as a chemical antioxidant, but do not all have the same vitamin E equivalence. Tocotrienols demonstrate activity depending on the type of antioxidant performance being measured. All tocotrienols have some physical antioxidant activity due to an ability to donate a hydrogen atom (a proton plus electron) from the hydroxyl group on the chromanol ring, to free radical and reactive oxygen species. Historically studies of tocotrienols account for less than 1% of all research into vitamin E. A scientific compilation of tocotrienol research, Tocotrienols: Vitamin E Beyond Tocopherols, was published in 2013.

Health effects

A number of health benefits of tocotrienols have been proposed, included decreased risk of heart disease and cancer. The Food and Nutrition Board of the Institute of Medicine of the United States National Academy of Sciences does not define a Recommended Dietary Allowance or Adequate Intake for tocotrienols.

Brain

A review of human studies in middle-aged and elderly stated "Evidence from prospective and case-control studies suggested that increased blood levels of tocotrienols were associated with favorable cognitive function outcomes." The review qualified this statement by noting that randomized, controlled clinical trials were needed to evaluate these observations.

Heart disease

Tocotrienols have been linked to improved markers of heart disease.

Diabetes

Animal models and observational studies in humans have shown potential benefit.

Side effects

Tocotrienols are generally well tolerated and without significant side effects.

History

The discovery of tocotrienols was first reported by Pennock and Whittle in 1964, describing the isolation of tocotrienols from rubber. The biological significance of tocotrienols was clearly delineated in the early 1980s, when its ability to lower cholesterol was first reported by Qureshi and Elson in the Journal of Medicinal Chemistry. During the 1990s, the anti-cancer properties of tocopherols and tocotrienols began to be delineated. The current commercial sources of tocotrienol are rice and palm. Other natural tocotrienol sources include rice bran oil, coconut oil, cocoa butter, barley, and wheat germ. Tocotrienols are safe and human studies show no adverse effects with consumption of 240 mg/day for 48 months. Tocotrienol rich fractions from rice, palm, or annatto, used in nutritional supplements, functional foods, and anti-aging cosmetics, are available in the market at 20%, 35%, 50%, and 70% total vitamin E content. 

Etymology

Tocotrienols are named by analogy to tocopherols (from Greek words meaning to bear a pregnancy; but with this word changed to include the chemical difference that tocotrienols are trienes, meaning that they share identical structure with the tocopherols except for the addition of the three double bonds to their side chains.

Comparison of tocotrienols and tocopherols

Tocotrienols have only a single chiral center, which exists at the 2' chromanol ring carbon, at the point where the isoprenoid tail joins the ring. The other two corresponding centers in the phytyl tail of the corresponding tocopherols do not exist as chiral centers for tocotrienols due to unsaturation (C-C double bonds) at these sites. Tocotrienols extracted from plants are always dextrorotatory stereoisomers, signified as d-tocotrienols. In theory, (levorotatory; l-tocotrienol) forms of tocotrienols could exist as well, which would have a 2S rather than 2R configuration at the molecules' single chiral center, but unlike synthetic, dl-alpha-tocopherol, the marketed tocotrienol dietary supplements are all d-tocotrienol extracts from palm or annatto oils.

Tocotrienol studies confirm anti-oxidation, anti-inflammatory potentials and suggest anti-cancer effects better than the common forms of tocopherol due to their chemical structure. Scientists have suggested tocotrienols are better antioxidants than tocopherols. It has been proposed that the unsaturated side-chain in tocotrienols causes them to penetrate tissues with saturated fatty layers more efficiently than tocopherol. Lipid ORAC values are highest for δ-tocotrienol. However that study also says: "Regarding α-tocopherol equivalent antioxidant capacity, no significant differences in the antioxidant activity of all vitamin E isoforms were found." 

Metabolism and bioavailability

The metabolism and thus the bioavailability of tocotrienols are not well understood and simply increasing the intake of tocotrienols might not increase tocotrienol levels in the body.

α-Tocopherol interference

Various studies have shown that alpha-tocopherol interferes with tocotrienol benefits. High levels of α-tocopherol increase cholesterol production. α-Tocopherol interference with tocotrienol absorption was described previously by Ikeda, who showed that α-tococopherol interfered with absorption of α-tocotrienol, but not γ-tocotrienol. Finally, α-tocopherol was shown to interfere with tocotrienols by increasing catabolism.

Sources

In nature, tocotrienols are present in many plants and fruits. The palm fruit (Elaeis guineensis) is particularly high in tocotrienols, primarily gamma-tocotrienol, alpha-tocotrienol and delta-tocotrienol. Other cultivated plants high in tocotrienols includes rice, wheat, barley, rye and oat. In annatto, tocotrienols are relatively abundant (only delta- and gamma-tocotrienol however) and it contains no tocopherols.

Research

Radiation countermeasures

Following exposure to gamma radiation, hematopoietic stem cells (HSCs) in the bone marrow, which are important for producing blood cells, rapidly undergo apoptosis (cell death). There are no known treatments for this acute effect of radiation. Two studies conducted by the U.S. Armed Forces Radiobiology Research Institute (AFRRI) found that treatment with γ-tocotrienol or δ-tocotrienol enhanced survival of hematopoietic stem cells, which are essential for renewing the body's supply of blood cells. Based on these successful results of studies in mice, γ-tocotrienol is being studied for its safety and efficacy as a radioprotective measure in nonhuman primates.