Hacker News Reader

Comment by jchw | Hacker News Reader

Not sure why your original comment disappeared.

I was kind of curious what the latencies might be for other contemporary processors/builds, and I'm not sure 70ns is actually really outside the normal margins.

Here's an i7-8700k build that is already pretty close to 70ns: https://www.userbenchmark.com/UserRun/18173216 - Also acknowledging that, this is not the 'best case' performance. But seems to be not so unusual either.

(edit: Removed section about latency ladder as I just realized it was caching and not system memory latency that we were most likely seeing initially, and therefore not terribly relevant.)

As far as I can tell, Zen 1 has similar latency characteristics[0][1]... so I guess I won't notice any degradation when upgrading to Zen 2.

[0]: https://www.userbenchmark.com/UserRun/18173307

[1]: https://www.userbenchmark.com/UserRun/18173240

View on HN

I will distill your post to: most user builds are bad balanced to begin with and they wont see a difference and would had a better price / more cores. Valid point, i agree with it. Still could be argued about a need for a better memory for Ryzen. This equalizes total build cost and you need to be informed about this platform trait beforehand, which will results in even worse average build balance. Imagine prebuilt PCs from wallmart with 2400 leftovers.

But my point is more about advanced user builds and press benchmarks.

[0] - 2400 cl16. It's much worse than 3600 cl16, used in referenced Ryzen 3000 test. It would be interesting to see how Ryzen would perform with it. May be zen2 comparison with 2400 vs 3400 (3600 is very rarely achievable there as IMC trait of the platform) would be a good illustration.

[1] - 3200 C16. But it performs way bellow expectations. Modern Intel chip should have ~55ns with it. With 3600 cl16 intel will have ~40ns.

You seem pretty certain this will manifest as a noticeable performance hit. Can this kind of thing be easily measured in practice? I don't know if my workloads would be memory latency sensitive and worse, I don't even have a clue how I would find out.

I'm not too concerned though, since I already use Zen 1 and it looks to be around the same. If I had to take a shot in the dark, I'm guessing it's just a consequence of the chiplet designs of Zen and Zen 2 that enabled them to scale so well. If it is such a tradeoff and there is not future gains to be made on latency for Zen platforms, I can accept that.

Though it does make me curious if Intel will ever stumble upon the same problems.

2400 vs 3000 vs 3200 MHz RAM | Ryzen 2nd gen: https://youtu.be/TjMq-Nv6Mq8

It affects general tasks too, but with much less magnitude than gaming, because games are concerned with frame times and overall latency the most.

Well, sure... but doesn't increasing the RAM clock increase the throughput as well?

What I'm asking is, is there a good way to test the effects of just memory latency?

You can manually increase the DRAM access latency in BIOS, leaving clock alone, and measure. It impacts throughput somewhat, but may be a little closer to an apples-to-apples comparison. I don't know that anyone has attempted to do this for video games on Zen 2 specifically (especially considering Zen 2 is only commercially available for the first time, like, today).

Here is a fresh Zen1 timings comparison from Reddit[1].

Most increase is between 3200cl14 vs 3200cl12. 12%. Difference between this two is almost purely a Latency.

Then compare 3200cl12 and 3600cl14 - 3%, marginally no increase. Almost no difference in latency, only throughput and IF.

Past 3200 RAM throughput and inter-core-communication (IF) has very little influence for Zen1 gaming. For Zen2 this scenario would differ in some ways but not too much.

[1]: https://www.reddit.com/r/Amd/comments/c9x8v7/2700x_memory_sc...

I think that linked list with randomly distributed nodes is a good benchmark. Each jump will be hit miss and cause load from RAM and you can't prefetch anything because next address depends on content of fetch. Performance of simple iteration of that linked list should correspond to random memory access performance.

> I don't know if my workloads would be memory latency sensitive and worse, I don't even have a clue how I would find out.

If you develop on something unix-ish valgrind's cachegrind will tell you about your L1 performance. On recent Linux you can get this straight from the kernel with `perf stat` https://perf.wiki.kernel.org/index.php/ (cache-misses are total misses in all levels)

The most basic question is: are you randomly accessing more then your processor's cache worth of memory?

You keep claiming this is going to ruin gaming performance based on napkin math. Why wouldn't you just reference ACTUAL benchmarks, which show your theory to be incorrect?

https://www.anandtech.com/show/14605/the-and-ryzen-3700x-390...

Reply on news.ycombinator.com