SMT and CMP Architectures

DINESH

INTRODUCTION

Contemporary forms of parallelism Instruction-level parallelism(ILP)

Wide-issue Superscalar processors (SS) 4 or more instruction per cycle Executing a single program or thread Attempts to find multiple instructions to issue each cycle. Out-of-order execution => instructions are sent to execution units based on

instruction dependencies rather than program order Thread-level parallelism(TLP)

Fine-grained multithreaded superscalars(FGMS) Contain hardware state for several threads Executing multiple threads On any given cycle a processor executes instructions from one of the

threads Multiprocessor(MP)

Performance improved by adding more CPUs

Simultaneous Multithreading

Key idea Issue multiple instructions from multiple threads each

cycle Features

Fully exploit thread-level parallelism and instruction-level parallelism.

Multiple functional units Modern processors have more functional units available then

a single thread can utilize. Register renaming and dynamic scheduling

Multiple instructions from independent threads can co-exist and co-execute.

Summary: Multithreaded Categories

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Superscalar Fine-Grained Coarse-GrainedSimultaneous Multithreading

Thread 1

Thread 2Thread 3Thread 4

Thread 5Idle slot

Horizontal dimension represents the instruction issue capabilty in each clock cycles.

Vertical dimension represents a sequence of clock cycles. Empty slots indicates that the corresponding issue slots

are unused in that clock cycles.

Superscalar processor with no multithreading:only one thread is processed in one clock cycle Use of issue slots is limited by a lack of ILP. Stalls such as an instruction cache miss leaves the entire processor

idle. Fine-grained multithreading:

switches threads on every clock cycle Pro: hide latency of from both short and long stalls Con: Slows down execution of the individual threads ready to go.

Only one thread issues inst. In a given clock cycle. Course-grained multithreading:

switches threads only on costly stalls (e.g., L2 stalls) Pros: no switching each clock cycle, no slow down for ready-to-

go threads. Reduces no of completely idle clock cycles. Con: limitations in hiding shorter stalls

Simultaneous Multithreading: exploits TLP at the same time it exploits ILP with multiple

threads using the issue slots in a single-clock cycle. issue slots is limited by the following factors:

Imbalances in the resource needs. Resource availability over multiple threads. Number of active threads considered. Finite limitations of buffer. Ability to fetch enough instructions from

multiple threads. Practical limitations of what instructions

combinations can issue from one thread and multiple threads.

Performance Implications of SMT Single thread performance is likely to go down (caches,

branch predictors, registers, etc. are shared) – this effect can be mitigated by trying to prioritize one thread

While fetching instructions, thread priority can dramatically influence total throughput – a widely accepted heuristic (ICOUNT): fetch such that each thread has an equal share of processor resources

With eight threads in a processor with many resources, SMT yields throughput improvements of roughly 2-4

Alpha 21464 and Intel Pentium 4 are examples of SMT

Effectively Using Parallelism on a SMT Processor Parallel workload

threads SS MP2 MP4 FGMT SMT

1 3.3 2.4 1.5 3.3 3.3

2 -- 4.3 2.6 4.1 4.7

4 -- -- 4.2 4.2 5.6

8 -- -- -- 3.5 6.1

Instruction Throughput executing a parallel workload

Comparison of SMT vs SuperscalarSMT processors are compared to base superscalarprocessors in several key measures : Utilization of functional units. Utilization of fetch units. Accuracy of branch predictor. Hit rates of primary caches. Hit rates of secondary caches.Performance improvement:• Issue slots.• Funtional units.• Renaming registers.

CMP Architecture Chip-level multiprocessing(CMP or multicore):

integrates two or more independent cores(normally a CPU) into a single package composed of a single integrated circuit(IC), called a die, or more dies packaged, each executing threads independently.

Every funtional units of a processor is duplicated. Multiple processors, each with a full set of

architectural resources, reside on the same die Processors may share an on-chip cache

or each can have its own cache Examples: HP Mako, IBM Power4 Challenges: Power, Die area (cost)

Single core computer

Single coreSingle core CPU chip

Multi-core CPU chipCore 1 Core 2 Core 3 Core 4

Chip Multithreading = Chip Multiprocessing + Hardware Multithreading.

Chip Multithreading is the capability of a processor to process multiple s/w threads simulataneous h/w threads of execution.

CMP is achieved by multiple cores on a single chip or multiple threads on a single core.

CMP processors are especially suited to server workloads, which generally have high levels of Thread-Level Parallelism(TLP).

Chip Multithreading

CMP’s PerformanceCMP’s are now the only way to build high performance

microprocessors , for a variety of reasons:o Large uniprocessors are no longer scaling in performance,

because it is only possible to extract a limited amount of parallelism from a typical instruction stream.

o Cannot simply ratchet up the clock speed on today’s processors,or the power dissipation will become prohibitive.

o CMT processors support many h/w strands through efficient sharing of on-chip resources such as pipelines, caches and predictors.

o CMT processors are a good match for server workloads,which have high levels of TLP and relatively low levels of ILP.

SMT and CMP• The performance race between SMT and CMP is not yet decided.• CMP is easier to implement, but only SMT has the ability to hide

latencies.• A functional partitioning is not exactly reached within a SMT

processor due to the centralized instruction issue.o A separation of the thread queues is a possible solution,

although it does not remove the central instruction issue.o A combination of simultaneous multithreading with the CMP

may be superior.• Research : combine SMT or CMP organization with the ability to

create threads with compiler support of fully dynamically out of a single thread.o Thread-level speculationo Close to multiscalar

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Multiprocessor vs. SMT

Multiprocessor(MP2) SMT

Unuti l i zedThread 1Thread 2

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