Memory-level parallelism (MLP) is a term in
computer architecture
In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, the ...
referring to the ability to have pending multiple
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered ...
operations, in particular
cache misses or
translation lookaside buffer
A translation lookaside buffer (TLB) is a memory cache that stores the recent translations of virtual memory to physical memory. It is used to reduce the time taken to access a user memory location. It can be called an address-translation cache. ...
(TLB) misses, at the same time.
In a single processor, MLP may be considered a form of
instruction-level parallelism
Instruction-level parallelism (ILP) is the parallel or simultaneous execution of a sequence of instructions in a computer program. More specifically ILP refers to the average number of instructions run per step of this parallel execution.
Dis ...
(ILP). However, ILP is often conflated with
superscalar
A superscalar processor is a CPU that implements a form of parallelism called instruction-level parallelism within a single processor. In contrast to a scalar processor, which can execute at most one single instruction per clock cycle, a sup ...
, the ability to execute more than one instruction at the same time, e.g. a processor such as the Intel
Pentium Pro
The Pentium Pro is a sixth-generation x86 microprocessor developed and manufactured by Intel and introduced on November 1, 1995. It introduced the P6 microarchitecture (sometimes termed i686) and was originally intended to replace the original ...
is five-way superscalar, with the ability to start executing five different microinstructions in a given cycle, but it can handle four different cache misses for up to 20 different load microinstructions at any time.
It is possible to have a machine that is not superscalar but which nevertheless has high MLP.
Arguably a machine that has no ILP, which is not superscalar, which executes one instruction at a time in a non-pipelined manner, but which performs hardware prefetching (not software instruction-level prefetching) exhibits MLP (due to multiple prefetches outstanding) but not ILP. This is because there are multiple memory ''operations'' outstanding, but not ''instructions''. Instructions are often conflated with operations.
Furthermore, multiprocessor and multithreaded computer systems may be said to exhibit MLP and ILP due to parallelism—but not intra-thread, single process, ILP and MLP. Often, however, we restrict the terms MLP and ILP to refer to extracting such parallelism from what appears to be non-parallel single threaded code.
See also
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Memory disambiguation
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Memory dependence prediction
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Hardware scout
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Runahead
References
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{{CPU technologies
Instruction processing
Parallel computing