Investigating Performance of Multi-Threading on Zen 3 and AMD Ryzen 5000
by Dr. Ian Cutress on December 3, 2020 10:00 AM EST- Posted in
- CPUs
- AMD
- Zen 3
- X570
- Ryzen 5000
- Ryzen 9 5950X
- SMT
- Multi-Threading
Conclusions: SMT On
I wasn’t too sure what we were going to see when I started this testing. I know the theory behind implementing SMT, and what it means for the instruction streams having access to core resources, and how cores that have SMT in mind from the start are built differently to cores that are just one thread per core. But theory only gets you so far. Aside from all the forum messages over the years talking about performance gains/losses when a product has SMT enabled, and the few demonstrations of server processors running focused workloads with SMT disabled, it is actually worth testing on real workloads to find if there is a difference at all.
Results Overview
In our testing, we covered three areas: Single Thread, Multi-Thread, and Gaming Performance.
In single threaded workloads, where each thread has access to all of the resources in a single core, we saw no change in performance when SMT is enabled – all of our workloads were within 1% either side.
In multi-threaded workloads, we saw an average uplift in performance of +22% when SMT was enabled. Most of our tests scored a +5% to a +35% gain in performance. A couple of workloads scored worse, mostly due to resource contention having so many threads in play – the limit here is memory bandwidth per thread. One workload scored +60%, a computational workload with little-to-no memory requirements; this workload scored even better in AVX2 mode, showing that there is still some bottleneck that gets alleviated with fewer instructions.
On gaming, overall there was no difference between SMT On and SMT Off, however some games may show differences in CPU limited scenarios. Deus Ex was down almost 10% when CPU limited, however Borderlands 3 was up almost 10%. As we moved to a more GPU limited scenario, those discrepancies were neutralized, with a few games still gaining single-digit percentage points improvement with SMT enabled.
For power and performance, we tested two examples where performance at two threads per core was either saw no improvement (Agisoft), or significant improvement (3DPMavx). In both cases, SMT Off mode (1 thread/core) ran at higher temperatures and higher frequencies. For the benchmark per performance was about equal, the power consumed was a couple of percentage points lower when running one thread per core. For the benchmark were running two threads per core has a big performance increase, the power in that mode was also lower, and there was a significant +91% performance per watt improvement by enabling SMT.
What Does This Mean?
I mentioned at the beginning of the article that SMT performance gains can be seen from two different viewpoints.
The first is that if SMT enables more performance, then it’s an easy switch to use, and some users consider that if you can get perfect scaling, then if SMT is an effective design.
The second is that if SMT enables too much performance, then it’s indicative of a bad core design. If you can get perfect scaling with SMT2, then perhaps something is wrong about the design of the core and the bottleneck is quite bad.
Having poor SMT scaling doesn’t always mean that the SMT is badly implemented – it can also imply that the core design is very good. If an effective SMT design can be interpreted as a poor core design, then it’s quite easy to see that vendors can’t have it both ways. Every core design has deficiencies (that much is true), and both Intel and AMD will tell its users that SMT enables the system to pick up extra bits of performance where workloads can take advantage of it, and for real-world use cases, there are very few downsides.
We’ve known for many years that having two threads per core is not the same as having two cores – in a worst case scenario, there is some performance regression as more threads try and fight for cache space, but those use cases seem to be highly specialized for HPC and Supercomputer-like tasks. SMT in the real world fills in the gaps where gaps are available, and this occurs mostly in heavily multi-threaded applications with no cache contention. In the best case, SMT offers a sizeable performance per watt increase. But on average, there are small (+22% on MT) gains to be had, and gaming performance isn’t disturbed, so it is worth keeping enabled on Zen 3.
Facebook
Twitter
RSS
126 Comments
View All Comments
GeoffreyA - Tuesday, December 8, 2020 - link
There's a single set of 4 decoders. In SMT mode, I believe some sharing is going in. This is from the original Zen design:https://images.anandtech.com/doci/10591/HC28.AMD.M...
GeoffreyA - Tuesday, December 8, 2020 - link
* going onnaive dev - Wednesday, December 9, 2020 - link
Right, I found that article as well and from that slide it looks like the decoder would be shared. But then that slide was from 2017, so that might have changed.It looks though as if the decoder could decode those 4 instructions from a single program counter only, right? It's not like the decoder could decode e.g. 2 instructions from program counter 1 and another 2 instructions from program counter 2?
GeoffreyA - Thursday, December 10, 2020 - link
I'm not too sure how the implementation works, but I expect they're shuffling both threads through the decoder at roughly the same time. The decoder has four units (I think 1 complex and 3 simple). As far as I'm aware, that has stayed the same in both Zen 2 and 3.mapesdhs - Thursday, December 10, 2020 - link
Ian, a question about Handbrake, though it may not apply to the type of test you used. I've read that Handbrake doing an h264 encode can only use 16 threads max. Does this mean that in theory one could run two separate h264 encodes on a 5950X and thus obtain a good overall throughput speedup? Have you tried such a thing? Or might this only work if it were possible to force one encode to only use the 16 threads of one 8c block (CCX?), and the other encode to use the rest? ie. so that the separate encodes are not fighting over the same cores or indeed the same CCX-shared L3? Is it possible to force this somehow? Also, if the claimed 16 thread limit for h264 is true, is there a performance difference for a single h264 encode between SMT on vs. off just in general? ie. with it on, is the OS smart enough to ensure that the 16 threads are spread across all the cores evenly rather than being scrunched onto fewer cores because reasons? If not, then turning SMT off might speed it up. Note that I'm using Windows for all this.I don't know if any of this applies to h265, but atm the encoding I do is still 1080p. I did an analysis of all available Ryzen CPUs based on performance, power consumption and cost (I ruled out Intel partly due to the latter two factors but also because of a poor platform upgrade path) and found that although the 5900X scored well, overall it was beaten by the 2700X, mainly because the latter is so much cheaper. However, the 5950X would look a lot better if one could run two encodes on it at the same time without clashing, but review articles naturally never try this. I wish I could test it, but the only 16c system I have is a dual-socket S2011 setup with two 2560 v2s, so the separate CPUs introduce all sorts of other issues (NUMA and suchlike).
I found something similar a long time ago when I noticed one could run six separate Maya frame renders on a 24-CPU SGI rack Onyx (essentially one render per CPU board), compared to running a single render on a quad-CPU (single board) deskside Onyx, giving a good overall throughput increase (the renderer being limited to 4 CPUs per job). See:
http://www.sgidepot.co.uk/perfcomp_RENDER4_maya1.h...
Funny actually, re what you say about an overly good speedup perhaps implying a less than optimal core design. Something odd about SGIs is how many times on a multi CPU system one can btain better results by using more threads than there are CPUs, baring in mind MIPS CPUs from that era did not have SMT, ie. the CPUs kinda behave as if they do have SMT even though they don't. I found this behaviour occured most for Blender and C-Ray.
So anyway, it would be great if it were possible to run two h264 encodes on a 5950X at the same time, but there's probably no point if the OS doesn't spread out the loads in a sensible manner, or if in that circumstance there isn't a way to force each encode to use a separate CCX.
All very specific to my use case of course, but I have hundreds of hours of material to convert, so the ability to get twice the throughput from a 5950X would make that CPU a lot more interesting; so far reviews I've read show it to be about 2x faster than the 2700X for h264 Handbrake (just one encode of course), but it costs 4.4x more, rather ruining the price/performance angle. And if it does work then I guess one could ask the same question of TR - could one run eight separate h264 encodes on a future Zen3 TR without the thread management being a total mess? :D I'm assuming it probably wouldn't be so good with the older Zen2 design given the split L3.
GeoffreyA - Sunday, December 13, 2020 - link
Interesting question. Would be nice if someone could give this a test on 16-core Ryzen or TR, and see what happens. Yesterday, I was able to take both FFmpeg and Handbrake up to 128 threads, and it does work; but, having only a 4-core, 4-thread CPU, can't comment.*As for x264's performance limit, I'm not sure at what number of threads it begins to flag; but, quality wise, using too many (say, over 16 at 1080p) is not advisable. According to the x264 developers, vertical resolution / threads shouldn't fall below 40-50 and certainly not below 30.
https://forum.doom9.org/showthread.php?p=1213185#p...
forum.doom9.org/showthread.php?p=1646307#post1646307
More posts on high core counts:
forum.doom9.org/showthread.php?t=173277
forum.doom9.org/showthread.php?t=175766
* As far as I know, Windows schedules threads all right. From 1903, on Zen 2, one CCX is supposed to be filled up, then another. I imagine 16 threads will be spread across two CCXs in the 5950X. FFmpeg's --threads switch could prove useful too.
GeoffreyA - Sunday, December 13, 2020 - link
-threads, not --threadsHere are links set out better (thought they'd link in the comment):
https://forum.doom9.org/showthread.php?p=1213185#p...
https://forum.doom9.org/showthread.php?p=1646307#p...
https://forum.doom9.org/showthread.php?t=173277
https://forum.doom9.org/showthread.php?t=175766
karthikpal - Friday, December 11, 2020 - link
Nice content bro<a href="https://www.tronicsmaster.com">Ryzen 7 5800X</a>
deil - Sunday, December 13, 2020 - link
I wonder when smt4 will hit the market a model with 3 copies of most things on the die, in a ring configuration fp/int/fp/int, cache inside a ring st would have a chance to use 2 FP modules for single int processor part (when others don't use it ofc).This kind of setup would have very interesting performance numbers at least. I am not saying it's a good idea, but interesting one for sure.
Machinus - Sunday, December 13, 2020 - link
This article omits one of the basic considerations in any manually-configured and custom-cooled desktop system: achieving uniform, preditcable thermal behavior. Unless you are building servers to perform only one or two specific types of mathematical operations, and can build, configure, and stress test on those instruction types alone, you need high confidence that the chip will never exceed the thermal flux densities of the cooling system you built. Fixed-clock systems with a static number of available cores have much more consistent thermal performance than chips whose clocks, and number of threads, are free-floating. This reduces your peak flops, but it significantly extends system lifetime. HEDT and HPC systems have double or triple-digit coure counts per sockrt in 2020; SMT is not worth paying the price of reduced hardware lifetime unless you are building extremely specialized calculation servers.