Investigating Performance of Multi-Threading on Zen 3 and AMD Ryzen 5000

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• quadibloc - Friday, December 4, 2020 - link

  On the one hand, if one program does a lot of integer calculations, and the other does a lot of floating-point calculations, putting them on the same core would seem to make sense because they're using different execution resources. On the other hand, if you use two threads from the same program on the same core, then you may have less contention for that core's cache memory.

• linuxgeex - Thursday, December 3, 2020 - link

  One of the key ways in which SMT helps keep the execution resources of the core active, is cache misses. When one thread is waiting 100+ clocks on reads from DRAM, the other thread can happily keep running out of cache. Of course, this is a two-edged sword. Since the second thread is consuming cache, the first thread was more likely to have a cache miss. So for the very best per-thread latency you're best off to disable SMT. For the very best throughput, you're best off to enable SMT. It all depends on your workload. SMT gives you that flexibility. A core without SMT lacks that flexibility.

• Dahak - Thursday, December 3, 2020 - link

  Now I only glanced through the article, but would it make more sense to use a lower core count cpu to see the benefits of SMT as using a higher core count might mean it will use the real core over the smt cores?

• Holliday75 - Thursday, December 3, 2020 - link

  Starting to think the entire point of the article was this subject is so damn complicated and hard to quantify for the average user that there is no point in trying unless you are running work loads in a lab environment to find the best possible outcome for the work you plan on doing. Who is going to bother doing that unless you work for a company where it makes sense to do so.

• 29a - Thursday, December 3, 2020 - link

  I wonder what the charts would look like with a dual core SMT processor. I think the game tests would have a good chance of changing. I'm a little surprised the author didn't think to test CPU's with more limited resources.

• Klaas - Thursday, December 3, 2020 - link

  Nice article. And nice benchmarks.
  
  Too bad you could not include more real-life test.
  I think that a processor with far less threads (like a 5600X)
  is easier to saturate with real-life software that actually uses threads.
  The Cache Sizes and Memory channels bandwith ratio to core is quite different
  for those 'smaller' processors. That will probably result in different benchmark results...
  So it would be interesting to see what those processors will do, SMT ON versus SMT OFF.
  I don't think the end result will be different, but it could even be a bigger victory for SMT ON.
  
  Another interesting area is virtualization.
  And as already mentioned in more comments it is very important that the Operating Systems
  will assign threads to the right core or SMT-Core combinations.
  That is even more important in virtualization situations...

• MDD1963 - Thursday, December 3, 2020 - link

  Determining the usefulness of SMT with 16 cores on tap is not quite as relevant as when this experiment might be done with, say, a 5600X or 5800X....; naturally 16 cores without SMT might still be plenty (as even 8 non SMT cores on the 9700K proved)

• thejaredhuang - Thursday, December 3, 2020 - link

  This would be a better test on a CPU that doesn't have 16 base cores. If you could try it on a 4C/8T part I think the difference would be more pronounced.

• dfstar - Thursday, December 3, 2020 - link

  The basic benefit of SMT is it allows the processer to hide the impact of long latency instructions on average IPC, since it can switch to new thread and execute those instructions. In this way it is similar to OOO(which leverages speculative execution to do the same) and also more flexible than fine-grained multi-threading. There is an overhead and cost(area/power) due to the duplicated structures in the core that will impact the perf/watt of pure single-threaded workloads, I don't think disabling SMT removes all this impact ...

• GreenReaper - Thursday, December 3, 2020 - link

  Perhaps not. But at the same time, it is likely that any non-SMT chip that has a SMT variant actually *is* a SMT chip, it is just disabled in firmware - either because it is broken on that chip, or because the non-SMT variant sold better.