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

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• abufrejoval - Thursday, December 3, 2020 - link

  It's hard to imagine a transistor defect that would break *only* SMT. As you say all non-SMT chips are really SMT chips internally and the decision to disable SMT doesn't really result in huge chunks of transistors going dark (the potential target area for physical defects).
  
  I'd say most of the SMT vs. no-SMT decisions on individual CPUs are binning related: SMT can create significantly more heat because there is less idle which allows the chip to cool. So if you have a chip with higher resistance in critical vias and require higher voltage to function, you need to sacrifice clocks, TDP or utilization (and permutations).

• leexgx - Saturday, December 5, 2020 - link

  With HT off I have definitely noticed less smoothness windows, as with HT it can keep the cpu active when a thread is slightly stuck

• iranterres - Thursday, December 3, 2020 - link

  Why are people still testing SMT in 2020? Cache coherency and hierarchy design is mature enough to offset the possible instruction bottleneck issues. I don't even know the purpose of this article at all... Anyways, perhaps fallng back to 2008? Come on...

• quadibloc - Friday, December 4, 2020 - link

  Well, instead of testing the concept of SMT, which has been around for a while, perhaps one could think of it as testing the implementation of SMT found on the chips we can get in 2020.

• eastcoast_pete - Friday, December 4, 2020 - link

  Thanks Ian! I always thought of SMT as a way of using whatever compute capacity a core has, but isn't being used in the moment. Hence it's efficient if many tasks need doing that each don't take a full core most of the time. However, that hits a snag if the cores get really busy. Hence (for desktop or laptop), 6 or 8 real cores are usually better than 4 cores that pretend to be 8.

• AntonErtl - Friday, December 4, 2020 - link

  I found the "Is SMT an good thing" discussion (and later discussion of the same topics) strange, because it seemed to take the POV of someone who wants to optimize some efficiency or utilization metric of someone who can choose the number of resources in the core. If you are in that situation, then the take of the EV8 designers was: we build a wide machine so that single-threaded applications can run fast, even though we know the wideness leads to low utilization; we also add SMT so that multi-threaded applications can increase utilization. Now, 20 years later, such wide cores become reality, although interestingly Apple and ARM do not add SMT.
  
  Anyway, buyers and users of off-the-shelf CPUs are not in that situation, and for them the questions are: For buyers: How much benefit does the SMT capabilty provide, and is it worth the extra money? For users: Does disabling SMT on this SMT-capable CPU increase the performance or the efficiency?
  
  The article shows that the answers to these questions depend on the application (although for the Zen3 CPUs available now the buyer's question does not pose itself).
  
  It would be interesting to see whether the wider Zen3 design gives significantly better SMT performance than Zen or Zen2 (and maybe also a comparison with Intel), but that would require also testing these CPUs.
  
  I did not find it surprising that the 5950X runs into the power limit with and without SMT. The resulting clock rates are mentioned in the text, but might be more interesting graphically than the temperature. What might also be interesting is the power consumed at the same clock frequency (maybe with fewer active cores and/or the clock locked at some lower clock rate).
  
  If SMT is so efficient (+91%) for 3DPMavx, why does the graphics only show a small difference?

• Bensam123 - Friday, December 4, 2020 - link

  Anand, while I value your in depth articles you guys really need to drop the 95th percentile frame times and get on board with 1% and .1% lows. What disrupts gaming the most is the hiccups, not looking at a statistically smooth chart. SMT/HT effects these THE most, especially in heavily single threaded games. If you aren't testing what it influences, why test it at all? Youtube reviews are also having problems with tests that don't reflect real world scenarios as well. Sometimes it's a lot more disagreeable then others.
  
  Completely invalid testing methodology at this point.
  
  My advice based on my own testing. You turn off SMT/HT except in scenarios in which you become CPU bound, across all cores, not one. This improved .1 and 1% frame time... IE stutters. You turn it on when you reach a point of 90%+ utilization as it helps and a lot when your CPU is maxed out. Generally speaking <6 and soon to be 8 cores should always have it on.
  
  You didn't even test where this helps the most and that's low end CPUs vs high end CPUs where you find the Windows scheduler messes things up.
  
  Also if you're testing this on your own, always turn it off in the bios. If you use something like process lasso or manually change affinity, windows will still put protected services and process onto those extra virtual cores causing contention issues that lead to the stuttering.
  
  Most obvious games that get a benefit from SMT/HT off are heavily single threaded games, such as MOBAS.

• Gloryholle - Friday, December 4, 2020 - link

  Testing Zen3 with 3200CL16?

• peevee - Friday, December 4, 2020 - link

  "Most modern processors, when in SMT-enabled mode, if they are running a single instruction stream, will operate as if in SMT-off mode and have full access to resources."
  
  Which would have access to the whole microinstruction cache (L0I) in SMT mode?

• Arbie - Friday, December 4, 2020 - link

  Another excellent AT article, which happens to hit my level of knowledge and interest; thanks!