Hardware Multithreading

1. Hardware Multithreading

2. 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

3. 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!

4. Hardware Multithreading • 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

5. CPU Support for Multithreading VA MappingA Address Translation VA MappingB Inst Cache Data Cache PCA PCB Fetch Logic Fetch Logic Decode Logic Fetch Logic Exec Logic Mem Logic Fetch Logic Write Logic RegA RegB

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

7. Coarse-Grain Multithreading

8. 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

9. Switch Threads on Icache miss • 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

10. Switch Threads on Dcache miss Abort 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

11. 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

12. Fine-Grain Multithreading

13. 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

14. Fine-Grain Multithreading Multithreading helps alleviate fine-grain dependencies (e.g. forwarding?)

15. I-cache misses in Fine Grain Multithreading Inst b is removed and the thread is marked as not ‘ready’ ‘Blue’ thread is not ready so ‘orange’ is executed An I-cache miss is overcome transparently

16. D-cache misses in Fine Grain Multithreading Thread marked as not ‘ready’. Remove Inst b. Update PC. ‘Blue’ thread is not ready so ‘orange’ is executed Mark the thread as not ‘ready’ and issue only from the other thread

17. D-cache misses in Fine Grain Multithreading In an out of order processor we may continue issuing instructions from both threads

18. 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

19. Simultaneous Multi-Threading

20. 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

21. Simultaneous MultiThreading Let’s look simply at instruction issue:

22. 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)

23. 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

24. 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

25. Hardware Multithreading

26. 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

27. 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, …

28. Comparison of Multithreading Techniques

29. 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?)