CPU Performance

For simplicity, we are listing the percentage performance differentials in all of our CPU testing – the number shown is the % performance of having SMT2 enabled compared to having the setting disabled. Our benchmark suite consists of over 120 tests, full details of which can be found in our #CPUOverload article.

Here are the single threaded results.

Single Threaded Tests
AMD Ryzen 9 5950X
AnandTech SMT Off
Baseline
SMT On 
y-Cruncher 100% 99.5%
Dwarf Fortress 100% 99.9%
Dolphin 5.0 100% 99.1%
CineBench R20 100% 99.7%
Web Tests 100% 99.1%
GeekBench (4+5) 100% 100.8%
SPEC2006 100% 101.2%
SPEC2017 100% 99.2%

Interestingly enough our single threaded performance was within a single percentage point across the stack (SPEC being +1.2%). Given that ST mode should arguably give more resources to each thread for consistency, the fact that we see no difference means that AMD’s implementation of giving a single thread access to all the resources even in SMT mode is quite good.

The multithreaded tests are a bit more diverse:

Multi-Threaded Tests
AMD Ryzen 9 5950X
AnandTech SMT Off
Baseline
SMT On
Agisoft Photoscan 100% 98.2%
3D Particle Movement 100% 165.7%
3DPM with AVX2 100% 177.5%
y-Cruncher 100% 94.5%
NAMD AVX2 100% 106.6%
AIBench 100% 88.2%
Blender 100% 125.1%
Corona 100% 145.5%
POV-Ray 100% 115.4%
V-Ray 100% 126.0%
CineBench R20 100% 118.6%
HandBrake 4K HEVC 100% 107.9%
7-Zip Combined 100% 133.9%
AES Crypto 100% 104.9%
WinRAR 100% 111.9%
GeekBench (4+5) 100% 109.3%

Here we have a number of different factors affecting the results.

Starting with the two tests that scored statistically worse with SMT2 enabled: yCruncher and AIBench. Both tests are memory-bound and compute-bound in parts, where the memory bandwidth per thread can become a limiting factor in overall run-time. yCruncher is arguably a math synthetic benchmark, and AIBench is still early-beta AI workloads for Windows, so quite far away from real world use cases.

Most of the rest of the benchmarks are between a +5% to +35% gain, which includes a number of our rendering tests, molecular dynamics, video encoding, compression, and cryptography. This is where we can see both threads on each core interleaving inside the buffers and execution units, which is the goal of an SMT design. There are still some bottlenecks in the system affecting both threads getting absolute full access, which could be buffer size, retire rate, op-queue limitations, memory limitations, etc – each benchmark is likely different.

The two outliers are 3DPM/3DPMavx, and Corona. These three are 45%+, with 3DPM going 66%+. Both of these tests are very light on the cache and memory requirements, and use the increased Zen3 execution port distribution to good use. These benchmarks are compute heavy as well, so splitting some of that memory access and compute in the core helps SMT2 designs mix those operations to a greater effect. The fact that 3DPM in AVX2 mode gets a higher benefit might be down to coalescing operations for an AVX2 load/store implementation – there is less waiting to pull data from the caches, and less contention, which adds to some extra performance.

Overall

In an ideal world, both threads on a core will have full access to all resources, and not block each other. However, that just means that the second thread looks like it has its own core completely. The reverse SMT method, of using one global core and splitting it into virtual cores with no contention, is known as VISC, and the company behind that was purchased by Intel a few years ago, but nothing has come of it yet. For now, we have SMT, and by design it will accelerate some key workloads when enabled.

In our CPU results, the single threaded benchmarks showed no uplift with SMT enabled/disabled in our real-world or synthetic workloads. This means that even in SMT enabled mode, if one thread is running, it gets everything the core has on offer.

For multi-threaded tests, there is clearly a spectrum of workloads that benefit from SMT.

Those that don’t are either hyper-optimized on a one-thread-per-core basis, or memory latency sensitive.

Most real-world workloads see a small uplift, an average of 22%. Rendering and ray tracing can vary depending on the engine, and how much bandwidth/cache/core resources each thread requires, potentially moving the execution bottleneck somewhere else in the chain. For execution limited tests that don’t probe memory or the cache at all, which to be honest are most likely to be hyper-optimized compute workloads, scored up to +77% in our testing.

Investigating SMT on Zen 3 Gaming Performance (Discrete GPU)
POST A COMMENT

126 Comments

View All Comments

  • warpuck - Friday, December 25, 2020 - link

    With a R 5 1600 it makes about 5-6% difference in usable clock speed. (200-250 Mhz) and also with temperature. With a R 7 3800X it is not as noticeable.
    If you reduce the background operations while gaming with either CPU.
    I don't know about recent game releases but older ones only use 2-4 cores (threads) so clocking the R 5 1600 @ 3750 (SMT on) Mhz vs 3975 Mhz (SMT off) does make a difference on frame rates
    Reply
  • whatthe123 - Saturday, December 5, 2020 - link

    it doesn't make much of a difference unless you go way past the TDP and have exotic cooling.

    these CPUs are already boosting close to their limits at stock settings to maintain high gaming performance.
    Reply
  • 29a - Saturday, December 5, 2020 - link

    There is a lot of different scenarios that would be interesting to see. I would like to see some testing with a dual core chip 2c/4t. Reply
  • Netmsm - Thursday, December 3, 2020 - link

    good point Reply
  • Wilco1 - Friday, December 4, 2020 - link

    I think that 5% area cost for SMT is marketing. If you only count the logic that is essential for SMT, then it might be 5%. However many resources need to be increased or doubled. Even if that helps single-threaded performance, it still adds a lot of area that you wouldn't need without SMT.

    Graviton 2 proves that 2 small non-SMT cores will beat one big SMT core on multithreaded workloads using a fraction of the silicon and power.
    Reply
  • peevee - Monday, December 7, 2020 - link

    Except they are not faster, but whatever. Reply
  • RickITA - Thursday, December 3, 2020 - link

    Several compute applications do not need hyper-threading. A couple of official references:
    1. Wolfram Mathematica: "Mathematica’s Parallel Computing suite does not necessarily benefit from hyper-threading, although certain kernel functionality will take advantage of it when it provides a speedup." [source: https://support.wolfram.com/39353?src=mathematica]. Indeed Mathematica automatically set-up a number of threads equal to the number of physical cores of the CPU.
    2. Intel MKV library. "Hyper-Threading Technology (HT Technology) is especially effective when each thread is performing different types of operations and when there are under-utilized resources on the processor. Intel MKL fits neither of these criteria as the threaded portions of the library execute at high efficiencies using most of the available resources and perform identical operations on each thread. You may obtain higher performance when using Intel MKL without HT Technology enabled." [source: https://software.intel.com/content/www/us/en/devel...].

    BTW Ian: Wolfram Mathematica has a benchmark mode [source: https://reference.wolfram.com/language/Benchmarkin...], please consider to add it to your test suite. Or something with Matlab.
    Reply
  • realbabilu - Thursday, December 3, 2020 - link

    Apparently intel mkl and Matlab that uses intel mkl only allowing AMD uses non AVX2 library only. Only Linux with fake cpu preloaded library could go around this.
    https://www.google.com/amp/s/simon-martin.net/2020...
    Reply
  • RickITA - Thursday, December 3, 2020 - link

    Not a matlab user, but this is no longer true since version 2020a. Source: https://www.extremetech.com/computing/308501-cripp... Reply
  • leexgx - Saturday, December 5, 2020 - link

    The "if not Intel genuine cpu" disabled all optimisations (this rubbish has been going on for years only 2020 or 2019 where they are actually fixing there code to detect if AVX is available, even BTRFS had this problem it wouldn't use hardware acceleration if it wasn't on an intel CPU, again lazy coding ) Reply

Log in

Don't have an account? Sign up now