Simultaneous Multithreading (SMT)

EECC722 - ShaabanEECC722 - Shaaban#1 Lec # 2 Fall 2004 9-8-2004

Simultaneous Multithreading (SMT)Simultaneous Multithreading (SMT)• An evolutionary processor architecture originally introduced in 1995 by Dean Tullsen

at the University of Washington that aims at reducing resource waste in wide issue processors.

• SMT has the potential of greatly enhancing superscalar processor computational capabilities by:

– Exploiting thread-level parallelism (TLP), simultaneously issuing, executing and retiring instructions from different threads during the same cycle.

– Providing multiple hardware contexts, hardware thread scheduling and context switching capability.

– Providing effective long latency hiding.


SMT IssuesSMT Issues• SMT CPU performance gain potential.• Modifications to Superscalar CPU architecture necessary to support SMT.• SMT performance evaluation vs. Fine-grain multithreading, Superscalar, Chip

Multiprocessors.• Hardware techniques to improve SMT performance:

– Optimal level one cache configuration for SMT.– SMT thread instruction fetch, issue policies.– Instruction recycling (reuse) of decoded instructions.

• Software techniques:– Compiler optimizations for SMT.– Software-directed register deallocation.– Operating system behavior and optimization.

• SMT support for fine-grain synchronization.• SMT as a viable architecture for network processors.• Current SMT implementation: Intel’s Hyper-Threading (2-way SMT) Microarchitecture

and performance in compute-intensive workloads.


Microprocessor Architecture TrendsMicroprocessor Architecture Trends

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CMPs

SMTSMT/CMPs


Evolution of MicroprocessorsEvolution of Microprocessors

Source: John P. Chen, Intel Labs


Microprocessor Frequency TrendMicroprocessor Frequency Trend

Result:Deeper PipelinesLonger stallsHigher CPI(lowers effective performance per cycle)

Frequency doubles each generation Number of gates/clock reduce by 25% Leads to deeper pipelines with more stages (e.g Intel Pentium 4E has 30+ pipeline stages)

Realty Check:Clock frequency scalingis slowing down!(Did silicone finally hit the wall?)

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Simultaneous Multithreading SMT:e.g. Intel’s Hyper-threading

Chip-Multiprocessors (CMPs)e.g IBM Power 4

Multiple micro-operations per cycle

Chip-LevelParallelProcessing

(Superscalar)


CPU Architecture Evolution:CPU Architecture Evolution:

Single Threaded/Issue PipelineSingle Threaded/Issue Pipeline

• Traditional 5-stage integer pipeline.• Increases Throughput: Ideal CPI = 1

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CPU Architecture Evolution:CPU Architecture Evolution:

Superscalar ArchitecturesSuperscalar Architectures• Fetch, issue, execute, etc. more than one instruction per cycle (CPI < 1).• Limited by instruction-level parallelism (ILP).


• Empty or wasted issue slots can be defined as either vertical waste or horizontal waste:

– Vertical waste is introduced when the processor issues no instructions in a cycle.

– Horizontal waste occurs when not all issue slots can be filled in a cycle.

Superscalar Architectures:Superscalar Architectures:Issue Slot Waste Classification


Sources of Unused Issue Cycles in an 8-issue Superscalar Processor.

Processor busy represents the utilized issue slots; allothers represent wasted issue slots.

61% of the wasted cycles are vertical waste, theremainder are horizontal waste.

Workload: SPEC92 benchmark suite.

Source: Simultaneous Multithreading: Maximizing On-Chip Parallelism Dean Tullsen et al., Proceedings of the 22rd Annual International Symposium on Computer Architecture, June 1995, pages 392-403.

SMT-1

Average 1.5instructions/cycleissue rate

Single-Threaded


Single-Threaded Superscalar Single-Threaded Superscalar Architectures:Architectures:All possible causes of wasted issue slots, and latency-hiding or latency reducing

traditional techniques that can reduce the number of cycles wasted by each

cause.


SMT-1


Fine-grain or Traditional Multithreaded ProcessorsFine-grain or Traditional Multithreaded Processors

• Multiple HW contexts (PC, SP, and registers).

• Only one context or thread issues instructions each cycle.

• Performance limited by Instruction-Level Parallelism (ILP) within each individual thread:– Can reduce some of the vertical issue slot waste.– No reduction in horizontal issue slot waste.

• Example Architecture: The Tera Computer System

Advanced CPU Architectures:Advanced CPU Architectures:


Fine-grain or Traditional Multithreaded ProcessorsFine-grain or Traditional Multithreaded Processors

The Tera Computer System• The Tera computer system is a shared memory multiprocessor

that can accommodate up to 256 processors.

• Each Tera processor is fine-grain multithreaded:

– Each processor can issue one 3-operation Long Instruction Word (LIW) every 3 ns cycle (333MHz) from among as many as 128 distinct instruction streams (hardware threads), thereby hiding up to 128 cycles (384 ns) of memory latency.

– In addition, each stream can issue as many as eight memory references without waiting for earlier ones to finish, further augmenting the memory latency tolerance of the processor.

– A stream implements a load/store architecture with three addressing modes and 31 general-purpose 64-bit registers.

– The instructions are 64 bits wide and can contain three operations: a memory reference operation (M-unit operation or simply M-op for short), an arithmetic or logical operation (A-op), and a branch or simple arithmetic or logical operation (C-op).

Source: http://www.cscs.westminster.ac.uk/~seamang/PAR/tera_overview.html


VLIW: Intel/HPVLIW: Intel/HP IA-64

Explicitly Parallel Instruction Computing (EPIC)Explicitly Parallel Instruction Computing (EPIC)

• Strengths: – Allows for a high level of instruction parallelism (ILP).– Takes a lot of the dependency analysis out of HW and places focus

on smart compilers.

• Weakness: – Limited by instruction-level parallelism (ILP) in a single thread.– Keeping Functional Units (FUs) busy (control hazards).– Static FUs Scheduling limits performance gains.– Resulting overall performance heavily depends on compiler

performance.



Single Chip MultiprocessorSingle Chip Multiprocessor• Strengths:

– Create a single processor block and duplicate.– Exploits Thread-Level Parallelism.– Takes a lot of the dependency analysis out of HW and places focus

on smart compilers.

• Weakness: – Performance within each processor still limited by individual

thread performance (ILP).– High power requirements using current VLSI processes.




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Single Chip Multiprocessor


SMT: Simultaneous Multithreading• Multiple Hardware Contexts running at the same time (HW context:

registers, PC, and SP etc.).

• Reduces both horizontal and vertical waste by having multiple threads keeping functional units busy during every cycle.

• Builds on top of current time-proven advancements in CPU design: superscalar, dynamic scheduling, hardware speculation, dynamic HW branch prediction, multiple levels of cache, hardware pre-fetching etc.

• Enabling Technology: VLSI logic density in the order of hundreds of millions of transistors/Chip.– Potential performance gain is much greater than the increase in

chip area and power consumption needed to support SMT.


SMT• With multiple threads running penalties from long-latency

operations, cache misses, and branch mispredictions will be hidden:– Reduction of both horizontal and vertical waste and thus

improved Instructions Issued Per Cycle (IPC) rate.

• Functional units are shared among all contexts during every cycle:

– More complicated register read and writeback stages.

• More threads issuing to functional units results in higher resource utilization.

• CPU resources may have to resized to accommodate the additional demands of the multiple threads running.– (e.g cache, TLBs, branch prediction tables, rename registers)


SMT: Simultaneous Multithreading

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Superscalar Traditional Multithreaded

Simultaneous Multithreading

Rows of squares represent instruction issue slotsBox with number x: instruction issued from thread xEmpty box: slot is wasted


SMT Performance ExampleSMT Performance Example

Inst Code Description Functional unitA LUI R5,100 R5 = 100 Int ALUB FMUL F1,F2,F3 F1 = F2 x F3 FP ALUC ADD R4,R4,8 R4 = R4 + 8 Int ALUD MUL R3,R4,R5 R3 = R4 x R5 Int mul/divE LW R6,R4 R6 = (R4) Memory portF ADD R1,R2,R3 R1 = R2 + R3 Int ALUG NOT R7,R7 R7 = !R7 Int ALUH FADD F4,F1,F2 F4=F1 + F2 FP ALUI XOR R8,R1,R7 R8 = R1 XOR R7 Int ALUJ SUBI R2,R1,4 R2 = R1 – 4 Int ALUK SW ADDR,R2 (ADDR) = R2 Memory port

• 4 integer ALUs (1 cycle latency)

• 1 integer multiplier/divider (3 cycle latency)

• 3 memory ports (2 cycle latency, assume cache hit)

• 2 FP ALUs (5 cycle latency)

• Assume all functional units are fully-pipelined


SMT Performance Example SMT Performance Example (continued)(continued)

Cycle Superscalar Issuing Slots SMT Issuing Slots1 2 3 4 1 2 3 4

1 LUI (A) FMUL (B) ADD (C) T1.LUI (A) T1.FMUL(B)

T1.ADD (C) T2.LUI (A)

2 MUL (D) LW (E) T1.MUL (D) T1.LW (E) T2.FMUL (B) T2.ADD (C)3 T2.MUL (D) T2.LW (E)45 ADD (F) NOT (G) T1.ADD (F) T1.NOT (G)6 FADD (H) XOR (I) SUBI (J ) T1.FADD (H) T1.XOR (I) T1.SUBI (J ) T2.ADD (F)7 SW (K) T1.SW (K) T2.NOT (G) T2.FADD (H)8 T2.XOR (I) T2.SUBI (J )9 T2.SW (K)

• 2 additional cycles for SMT to complete program 2• Throughput:

– Superscalar: 11 inst/7 cycles = 1.57 IPC

– SMT: 22 inst/9 cycles = 2.44 IPC

– SMT is 2.44/1.57 = 1.55 times faster than superscalar for this example


Modifications to Superscalar CPUs Modifications to Superscalar CPUs Necessary to support SMTNecessary to support SMT

• Multiple program counters and some mechanism by which one fetch unit selects one each cycle (thread instruction fetch policy).

• A separate return stack for each thread for predicting subroutine return destinations.

• Per-thread instruction retirement, instruction queue flush, and trap mechanisms.

• A thread id with each branch target buffer entry to avoid predicting phantom branches.

• A larger register file, to support logical registers for all threads plus additional registers for register renaming. (may require additional pipeline stages).

• A higher available main memory fetch bandwidth may be required.• Larger data TLB with more entries to compensate for increased virtual to

physical address translations.

• Improved cache to offset the cache performance degradation due to cache sharing among the threads and the resulting reduced locality.

– e.g Private per-thread vs. shared L1 cache.

Source: Exploiting Choice: Instruction Fetch and Issue on an Implementable Simultaneous Multithreading Processor,Dean Tullsen et al. Proceedings of the 23rd Annual International Symposium on Computer Architecture, May 1996, pages 191-202.

SMT-2


Current Implementations of SMTCurrent Implementations of SMT• Intel’s recent implementation of Hyper-Threading

Technology (2-thread SMT) in its current P4 Xeon processor family represent the first and only current implementation of SMT in a commercial microprocessor.

• The Alpha EV8 (4-thread SMT) originally scheduled for production in 2001 is currently on indefinite hold :(

• Current technology has the potential for 4-8 simultaneous threads:

– Based on transistor count and design complexity.


A Base SMT Hardware Architecture.

Source: Exploiting Choice: Instruction Fetch and Issue on an Implementable Simultaneous Multithreading Processor,

Dean Tullsen et al. Proceedings of the 23rd Annual International Symposium on Computer Architecture, May 1996, pages 191-202.

SMT-2


Example SMT Vs. Superscalar PipelineExample SMT Vs. Superscalar Pipeline

• The pipeline of (a) a conventional superscalar processor and (b) that pipeline modified for an SMT processor, along with some implications of those pipelines.


Dean Tullsen et al. Proceedings of the 23rd Annual International Symposium on Computer Architecture, May 1996, pages 191-202.

Based on the Alpha 21164

SMT-2

Two extra pipeline stages added for reg. Read/write to account for the size increase of the register file


Intel Xeon Processor PipelineIntel Xeon Processor Pipeline

Source: Intel Technology Journal , Volume 6, Number 1, February 2002. SMT-8


Intel Xeon Out-of-order Execution Intel Xeon Out-of-order Execution Engine Detailed Pipeline Engine Detailed Pipeline

Source: Intel Technology Journal , Volume 6, Number 1, February 2002. SMT-8


SMT Performance ComparisonSMT Performance Comparison• Instruction throughput from simulations by Eggers et al. at The University of Washington, using both multiprogramming and parallel workloads:

Multiprogramming workload

Superscalar Traditional SMTThreads Multithreading 1 2.7 2.6 3.1 2 - 3.3 3.5 4 - 3.6 5.7 8 - 2.8 6.2

Parallel Workload

Superscalar MP2 MP4 Traditional SMTThreads Multithreading 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


• The following machine models for a multithreaded CPU that can issue 8 instruction per cycle differ in how threads use issue slots and functional units:

• Fine-Grain Multithreading:– Only one thread issues instructions each cycle, but it can use the entire issue width of the

processor. This hides all sources of vertical waste, but does not hide horizontal waste. • SM:Full Simultaneous Issue.

– This is a completely flexible simultaneous multithreaded superscalar: all eight threads compete for each of the 8 issue slots each cycle. This is the least realistic model in terms of hardware complexity, but provides insight into the potential for simultaneous multithreading. The following models each represent restrictions to this scheme that decrease hardware complexity.

• SM:Single Issue,SM:Dual Issue, and SM:Four Issue:– These three models limit the number of instructions each thread can issue, or have active in the

scheduling window, each cycle.

– For example, in a SM:Dual Issue processor, each thread can issue a maximum of 2 instructions per cycle; therefore, a minimum of 4 threads would be required to fill the 8 issue slots in one cycle.

• SM:Limited Connection. – Each hardware context is directly connected to exactly one of each type of functional unit.

– For example, if the hardware supports eight threads and there are four integer units, each integer unit could receive instructions from exactly two threads.

– The partitioning of functional units among threads is thus less dynamic than in the other models, but each functional unit is still shared (the critical factor in achieving high utilization).

Possible Machine Models for an 8-way Multithreaded Processor


SMT-1


Comparison of Multithreaded CPU Models Complexity

A comparison of key hardware complexity features of the various models (H=high complexity).

The comparison takes into account:– the number of ports needed for each register file, – the dependence checking for a single thread to issue multiple instructions,– the amount of forwarding logic, – and the difficulty of scheduling issued instructions onto functional units.


SMT-1


Simultaneous Vs. Fine-Grain Multithreading Performance

Instruction throughput as a function of the number of threads. (a)-(c) show the throughput by thread priority for particular models, and (d) shows the total throughput for all threads for each of the six machine models. The lowest segment of each bar is the contribution of the highest priority thread to the total throughput.


SMT-1

Workload:SPEC92


• Results for the multiprocessor MP vs. simultaneous multithreading SM comparisons.The multiprocessor always has one functional unit of each type per processor. In most cases the SM processor has the same total number of each FU type as the MP.

Simultaneous Multithreading Vs. Single-Chip Multiprocessing


SMT-1


Impact of Level 1 Cache Sharing on SMT PerformanceImpact of Level 1 Cache Sharing on SMT Performance • Results for the simulated cache configurations, shown relative to the

throughput (instructions per cycle) of the 64s.64p

• The caches are specified as:

[total I cache size in KB][private or shared].[D cache size][private or shared]

For instance, 64p.64s has eight private 8 KB I caches and a shared 64 KB data


SMT-1

Best overall performance of configurations consideredachieved by 64s.64s(64K data cache shared64K instruction cache shared)


The Impact of Increased Multithreading on Some Low LevelMetrics for Base SMT Architecture


SMT-2


Possible SMT Thread Instruction Fetch Scheduling PoliciesPossible SMT Thread Instruction Fetch Scheduling Policies• Round Robin:

– Instruction from Thread 1, then Thread 2, then Thread 3, etc. (eg RR 1.8 : each cycle one thread fetches up to eight instructions

RR 2.4 each cycle two threads fetch up to four instructions each)

• BR-Count: – Give highest priority to those threads that are least likely to be on a wrong path

by by counting branch instructions that are in the decode stage, the rename stage, and the instruction queues, favoring those with the fewest unresolved branches.

• MISS-Count:– Give priority to those threads that have the fewest outstanding Data cache misses.

• ICount:– Highest priority assigned to thread with the lowest number of instructions in

static portion of pipeline (decode, rename, and the instruction queues).

• IQPOSN:– Give lowest priority to those threads with instructions closest to the head of either

the integer or floating point instruction queues (the oldest instruction is at the head of the queue).


SMT-2


Instruction Throughput For Round Robin Instruction Fetch Scheduling


SMT-2

Best overall instruction throughput achieved using round robin RR.2.8(in each cycle two threads each fetch a block of 8 instructions)

Workload: SPEC92


Instruction throughput & Thread Fetch Policy


Dean Tullsen et al. Proceedings of the 23rd Annual International Symposium on Computer Architecture, May 1996, pages 191-202. SMT-2

Workload: SPEC92

All other fetch heuristics provide speedup over round robinInstruction Count ICOUNT.2.8 provides most improvement5.3 instructions/cycle vs 2.5 for unmodified superscalar.


Low-Level Metrics For Round Robin 2.8, Icount 2.8

ICOUNT improves on the performance of Round Robin by 23%by reducing IQ clog by selecting a better mix of instructions to queue


Possible SMT Instruction Issue PoliciesPossible SMT Instruction Issue Policies• OLDEST FIRST: Issue the oldest instructions (those

deepest into the instruction queue, the default).

• OPT LAST and SPEC LAST: Issue optimistic and speculative instructions after all others have been issued.

• BRANCH FIRST: Issue branches as early as possible in order to identify mispredicted branches quickly.

Source: Exploiting Choice: Instruction Fetch and Issue on an Implementable Simultaneous Multithreading Processor,Dean Tullsen et al. Proceedings of the 23rd Annual International Symposium on Computer Architecture, May 1996, pages 191-202. SMT-2

Instruction issue bandwidth is not a bottleneck in SMT as shown above


RIT-CE SMT Project GoalsRIT-CE SMT Project Goals• Investigate performance gains from exploiting Thread-

Level Parallelism (TLP) in addition to current Instruction-Level Parallelism (ILP) in processor design.

• Design and simulate an architecture incorporating Simultaneous Multithreading (SMT) including OS interaction (LINUX-based kernel?).

• Study operating system and compiler optimizations to improve SMT processor performance.

• Performance studies with various workloads using the simulator/OS/compiler:– Suitability for fine-grained parallel applications?– Effect on multimedia applications?


RIT-CE SMT Project Project ChartRIT-CE SMT Project Project Chart

S im u la to r Com piler

Linker/Loader

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S y s te m C a ll Pro x y(O S s pe cif ic)

Kernel Code

S im ulator w ill repres enthardw are w ith kernelc ontext

Kernel Code w illprovide the threadthat w ill be heldin the HW kernelc ontext

Com piler is s im ply ahac ked vers ion gc c(us ing as s em bler from hos ts ys tem )

P roc es s M anagem entM em ory M anagem ent

S M T Kernel S im ulation


Simulator (sim-SMT) @ RIT CESimulator (sim-SMT) @ RIT CE• Execution-driven, performance simulator.• Derived from Simple Scalar tool set.• Simulates cache, branch prediction, five pipeline stages• Flexible:

– Configuration File controls cache size, buffer sizes, number of functional units.• Cross compiler used to generate Simple Scalar assembly language.• Binary utilities, compiler, and assembler available.• Standard C library (libc) has been ported.• Sim-SMT Simulator Limitations:Sim-SMT Simulator Limitations:

– Does not keep precise exceptions.– System Call’s instructions not tracked.– Limited memory space:

• Four test programs’ memory spaces running on one simulator memory space

• Easy to run out of stack space


Simulator Memory Address SpaceSimulator Memory Address Space


sim-SMTsim-SMT Simulation Runs & Results Simulation Runs & Results• Test Programs used:

– Newton interpolation.– Matrix Solver using LU decomposition.– Integer Test Program.– FP Test Program.

• Simulations of a single program– 1,2, and 4 threads.

• System simulations involve a combination of all programs simultaneously– Several different combinations were run

• From simulation results:– Performance increase:

• Biggest increase occurs when changing from one to two threads.

– Higher issue rate, functional unit utilization.


Performance (IPC)Performance (IPC)Simulation Results:Simulation Results:


Simulation Results: Simulation Results: Simulation Time Simulation Time


Instruction Issue RateInstruction Issue RateSimulation Results:Simulation Results:


Performance Vs. Issue BWPerformance Vs. Issue BWSimulation Results:Simulation Results:


Functional Unit UtilizationFunctional Unit UtilizationSimulation Results:Simulation Results:


No Functional Unit AvailableNo Functional Unit AvailableSimulation Results:Simulation Results:


Horizontal Waste RateHorizontal Waste RateSimulation Results:Simulation Results:


Vertical Waste RateVertical Waste RateSimulation Results:Simulation Results:


SMT: Simultaneous Multithreading• Strengths:

– Overcomes the limitations imposed by low single thread instruction-level parallelism.

– Multiple threads running will hide individual control hazards (branch mispredictions).

• Weaknesses: – Additional stress placed on memory hierarchy Control unit

complexity.– Sizing of resources (cache, branch prediction, TLBs etc.)– Accessing registers (32 integer + 32 FP for each HW context):

• Some designs devote two clock cycles for both register reads and register writes.

Documents

Simultaneous Multithreading (SMT)