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

126 Comments

View All Comments

• MrSpadge - Friday, December 4, 2020 - link

  That doesn't mean SMT is mainly responsible for that. The x86 decoders are a lot more complex. And at the top end you get diminishing performance returns for additional die area.

• Wilco1 - Friday, December 4, 2020 - link

  I didn't say all the difference comes from SMT, but it can't be the x86 decoders either. A Zen 2 without L2 is ~2.9 times the size of a Neoverse N1 core in 7nm. That's a huge factor. So 2 N1 cores are smaller and significantly faster than 1 SMT2 Zen 2 core. Not exactly an advertisement for SMT, is it?

• Dolda2000 - Friday, December 4, 2020 - link

  >Graviton 2 gives 75-80% of the performance of the fastest Rome at less than a third of the area
  To be honest, it wouldn't surprise me one bit if 90% of the area gives 10% of the performance. Wringing out that extra 1% single-threaded performance here or there is the name of the game nowadays.
  
  Also there are many other differences that probably cost a fair bit of silicon, like wider vector units (NEON is still 128-bit, and exceedingly few ARM cores implement SVE yet).

• Wilco1 - Saturday, December 5, 2020 - link

  It's nowhere near as bad with Arm designs showing large gains every year. Next generation Neoverse has 40+% higher IPC.
  
  Yes Arm designs opt for smaller SIMD units. It's all about getting the most efficient use out of transistors. Having huge 512-bit SIMD units add a lot of area, power and complexity with little performance gain in typical code. That's why 512-bit SVE is used in HPC and nowhere else.
  
  So with Arm you get many different designs that target specific markets. That's more efficient than one big complex design that needs to address every market and isn't optimal in any.

• whatthe123 - Saturday, December 5, 2020 - link

  The extra 20% of performance is difficult to achieve. You can already see it on zen CPUs, where 16 core designs are dramatically more efficient per core in multithread running at around 3.4ghz, vs 8 core designs running at 4.8ghz. I've always hated these comparisons with ARM for this reason... you need a part with 1:1 watt parity to make a fair comparison, otherwise 80% performance at half the power can also be accomplished even on x86 by just reducing frequency and upping core count.

• Wilco1 - Saturday, December 5, 2020 - link

  Graviton clocks low to conserve power, and still gets close to Rome. You can easily clock it higher - Ampere Altra does clock the same N1 core 32% higher. So that 20-25% gap is already gone. We also know about the next generation (Neoverse N2 and V1) which have 40+% higher IPC.
  
  Yes adding more cores and clocking a bit lower is more efficient. But that's only feasible when your core is small! Altra Max has 128 cores on a single die, and I don't think we'll see AMD getting anywhere near that in the next few years even with chiplets.

• peevee - Monday, December 7, 2020 - link

  It is obviously a lot LESS than 5%. Nothing that matters in terms of transistors (caches and vector units) increases. Even doubling of registers would add a few hundreds/thousands of transistors on a chip with tens of billions of transistors, less than 0.000001%.
  
  They can double all scalar units and it still would be below 1% increase.

• Kangal - Friday, December 4, 2020 - link

  I agree.
  Adding SMT/HT requires something like a +10% increase in the Silicon Budget, and a +5% increase in power draw but increases performance by +30%, speaking in general. So it's worth the trade-off for daily tasks, and those on a budget.
  
  What I was curious to see, is if you disabled SMT on the 5950X, which has lots of cores. Leaving each thread with slightly more resources. And use the extra thermals to overclock the processor. How would that affect games?
  
  My hunch?
  Thread-happy games like Ashes of Singularity would perform worse, since it is optimised and can take advantage of the SMT. Unoptimized games like Fallout 76 should see an increase in performance. Whereas actually optimised games like Metro Exodus they should be roughly equal between OC versus SMT.
  

• Dolda2000 - Friday, December 4, 2020 - link

  >What I was curious to see, is if you disabled SMT on the 5950X, which has lots of cores.
  That is exactly what he did in this article, though.

• Kangal - Saturday, December 5, 2020 - link

  I guess you didn't understand my point.
  Think of a modern game which is well optimised, is both GPU intensive and CPU intensive. Such as Far Cry V or Metro Exodus. These games scale well anywhere from 4-physical-core to 8-physical-cores.
  
  So using the 5950X with its 16-physical-cores, you really don't need extra threads. In fact, it's possible to see a performance uplift without SMT, dropping it from 32-shared-threads down to 16-full-threads, as each core gets better utilisation. Now add to that some overclocking (+0.2GHz ?) due to the extra thermal headroom, and you may legitimately get more performance from these titles. Though I suspect they wouldn't see any substantial increases or decreases in frame rates.
  
  In horribly optimised games, like Fallout 76, Mafia 3, or even AC Odyssey, anything could happen (though probably they would see some increases). Whereas we already know that in games that aren't GPU intensive, but CPU intensive (eg practically all RTS games), these were designed to scale up much much better. So even with the full-cores and overclock, we know these games will actually show a decrease in performance from losing those extra threads/SMT.