Hardware Multithreading

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Increasing CPU Performance

• By increasing clock frequency• By increasing Instructions per Clock• Minimizing memory access impact – data cache• Maximising Inst issue rate – branch prediction• Maximising Inst issue rate – superscalar• Maximising pipeline utilisation – avoid instruction

dependencies – out of order execution

Increasing Parallelism

• Amount of parallelism that we can exploit is limited by the programs– Some areas exhibit great parallelism– Some others are essentially sequential

• In the later case, where can we find additional independent instructions?– In a different program!

• Allow multiple threads to share a single processor

• Requires replicating the independent state of each thread

• Virtual memory can be used to share memory among threads

CPU Support for Multithreading

Data Cache

Fetch Logic

Decode Logic

Fetch Logic

Exec Logic

Fetch Logic

Write Logic

Inst Cache

VA MappingA

VA MappingB

AddressTranslation

• Different ways to exploit this new source of parallelism– Coarse-grain parallelism– Fine-grain parallelism– Simultaneous Multithreading

Coarse-Grain Multithreading

• Issue instructions from a single thread • Operate like a simple pipeline

• Switch Thread on “expensive” operation:– E.g. I-cache miss– E.g. D-cache miss

Switch Threads on Icache miss1 2 3 4 5 6 7

Inst a IF ID EX MEM WB

Inst b IF ID EX MEM WB

Inst c IF MISS ID EX MEM WB

Inst d IF ID EX MEM

Inst e IF ID EX

Inst f IF ID

Inst X

Inst Y

Inst Z

- - - -

• Remove Inst c and switch to other thread• The next thread will continue its execution

until there is another I-cache or D-cache miss

Switch Threads on Dcache miss1 2 3 4 5 6 7

Inst a IF ID EX M-Miss WB

Inst c IF ID EX MEM WB

Inst d IF ID EX MEM

Inst e IF ID EX

Inst f IF ID

MISS MISS MISS

Inst X

Inst Y

Abort theseAbort these

• Remove Inst a and switch to other thread– Remove the rest of instructions from ‘blue’ thread– Roll back ‘blue’ PC to point to Inst a

Coarse Grain Multithreading

• Good to compensate for infrequent, but expensive pipeline disruption

• Minimal pipeline changes– Need to abort all the instructions in “shadow” of

Dcache miss overhead– Resume instruction stream to recover

• Short stalls (data/control hazards) are not solved

Fine-Grain Multithreading

• Overlap in time the execution of several threads

• Usually using Round Robin among all the threads in a ‘ready’ state

• Requires instantaneous thread switching

Fine-Grain Multithreading

• Multithreading helps alleviate fine-grain dependencies (e.g. forwarding?)

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Inst M IF ID EX MEM WB

Inst N IF ID EX MEM

Inst c IF ID EX

Inst P IF ID

I-cache misses in Fine Grain Multithreading

• An I-cache miss is overcome transparently

1 2 3 4 5 6 7

Inst b IF-MISS - - - -

Inst N IF ID EX MEM

Inst P IF ID EX

Inst Q IF ID

Inst b is removed and the thread is marked as not ‘ready’

‘Blue’ thread is not ready so ‘orange’ is executed

D-cache misses in Fine Grain Multithreading

• Mark the thread as not ‘ready’ and issue only from the other thread

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Inst a IF ID EX M-MISS Miss Miss WB

Inst b IF ID - - -

Inst N IF ID EX MEM

Inst P IF ID EX

Inst Q IF ID

Thread marked as not ‘ready’. Remove Inst b. Update PC.

‘Blue’ thread is not ready so ‘orange’ is executed

1 2 3 4 5 6 7

Inst a IF RO EX MEM WB

Inst M IF RO EX MEM WB

Inst N IF ID EX MEM

Inst c IF ID EX

Inst P IF ID

D-cache misses in Fine Grain Multithreading

• In an out of order processor we may continue issuing instructions from both threads

4 5 6 7

M MISS

EX MEM WB

IF ID EX MEM

4 5 6 7

M MISS Miss Miss WB

EX MEM WB

RO (RO) (RO) EX

IF RO EX MEM

IF (RO) (RO)

Fine Grain Multithreading

• Improves the utilisation of pipeline resources• Impact of short stalls is alleviated by executing

instructions from other threads

• Single thread execution is slowed• Requires an instantaneous thread switching

mechanism

Simultaneous Multi-Threading

• The main idea is to exploit instructions level parallelism and thread level parallelism at the same time

• In a superscalar processor issue instructions from different threads

• Instructions from different threads can be using the same stage of the pipeline

Simultaneous MultiThreading

• Let’s look simply at instruction issue:1 2 3 4 5 6 7 8 9 10

Inst N IF ID EX MEM WB

Inst c IF ID EX MEM WB

Inst P IF ID EX MEM WB

Inst Q IF ID EX MEM WB

Inst d IF ID EX MEM WB

Inst e IF ID EX MEM WB

Inst R IF ID EX MEM WB

SMT issues

• Asymmetric pipeline stall– One part of pipeline stalls – we want other

pipeline to continue• Overtaking – want unstalled thread to make

progress• Existing implementations on O-o-O, register

renamed architectures (similar to tomasulo)

SMT: Glimpse Into The Future?

• Scout threads?– A thread to prefetch memory – reduce cache miss

overhead

• Speculative threads?– Allow a thread to execute speculatively way past

branch/jump/call/miss/etc– Needs revised O-o-O logic– Needs and extra memory support– See Transactional Memory

Simultaneous Multi Threading

• Extracts the most parallelism from instructions and threads

• Implemented only in out-of-order processors because they are the only able to exploit that much parallelism

• Has a significant hardware overhead

Benefits of Hardware Multithreading

• All multithreading techniques improve the utilisation of processor resources and, hence, the performance

• If the different threads are accessing the same input data they may be using the same regions of memory – Cache efficiency improves in these cases

Disadvantages of Hardware Multithreading

• The perceived performance may be degraded when comparing with a single-thread CPU– Multiple threads interfering with each other

• The cache has to be shared among several threads so effectively they would use a smaller cache

• Thread scheduling at hardware level adds high complexity to processor design– Thread state, managing priorities, OS-level

information, …

Comparison of Multithreading Techniques

1 2 1 2 3 1 2 3 13 4 54 5 6 6 2 3 47 8 7 4 5 5 69 10 11 12 8 6 7 8

7 9 10 11 129 10 11 12 8 9 1013 14

13 15 1614 15 16

1 2 1 2 1 2 1 23 1 2 3 3 1 2 34 5 6 1 2 3 1 3 4 57 8 1 4 5 6 69 10 11 12 3 2 3 4 7

4 5 7 8 3 51 2 3 4 5 5 6 8 64 5 2 3 4 9 10 11 126 4 5 6 7 8 77 6 9 10 11 12

e ——

——

e ——

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Coarse-grain Fine Grain SMT

Thread A Thread B Thread C Thread D

Multithreading Summary

• A cost-effective way of finding additional parallelism for the CPU pipeline

• Available in x86, Itanium, Power and SPARC• (Most architectures) Present additional CPU

thread as additional CPU to Operating System• Operating Systems Beware!!! (why?)

Hardware Multithreading

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