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Comment by throwaway84742 | Hacker News Reader

I’m more impressed by Xeon Gold vs EPYC perf. The fact that Power 9 is currently slower on workloads that heavily benefit from hand-optimized code specific to Intel CPUs should not really surprise anybody. It’s likely that this code just uses a “portable”, slow implementation on Power9, just because nobody has the systems to work with yet. Additionally it’s also likely that most software in these tests can’t take advantage of such a huge number of threads.

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> I’m more impressed by Xeon Gold vs EPYC perf.

The hardware used in this benchmark isn't very well chosen. All the systems have a different number of cores (Power 16, AMD 32, Intel 40), and the AMD system has the additional disadvantage over the others of being 1P instead of 2P, which approximately halves the available cooling and correspondingly limits clock speeds. The results aren't very surprising given that.

Making comparisons like this can make sense when one of the competitors doesn't make an equivalent product, but they all do make products with an equivalent number of sockets and cores.

I believe the fair comparison would be performance per dollar - comparing similarly priced components. Same sockets and cores can have wildly different costs.

Per dollar of CPU, CPU + power, CPU + motherboard, TCO?

Comparing like with like can give you a better idea of what's going on even if the prices aren't the same. It's a matter of isolating the architecture from the other factors. If a 40 core 2P Intel system is faster than a 32 core 1P AMD system, you don't know if the difference is architectural or just the extra cores and sockets, so you don't know whether to expect a 48 core 2P AMD system to be faster or slower than the Intel system.

And the tested systems aren't equivalently priced here either.

I still need to see raw performance: single core, multi-core single socket, and multi-socket.

It depends on what you want to compare. If it's performance at a certain price point, you select systems at that point. You can compare on performance per watt or per volume or more abstract comparisons (ideally unbound by price or size) of performance per core, socket or thread at various load factors.

Then test against 2 EPYC 7451s as well for a fairer comparison.

Also, some tuning for the POWER software would probably be very welcome.

I wonder how 1P vs 2P would affect memory bandwidth. The 1P vs 2P difference would probably be deeper than just CPU clock speed.

Also, I wonder if these were Meltdown and Spectre patched.

If you are memory throughput bound, and both processors are working independently (think embarrassingly parallelizable problems) you'd probably benefit from the total throughput change.

Otherwise it's pretty easy to get worse performance due to numa issues

EPYC is already four NUMA nodes per socket though.

> the AMD system has the additional disadvantage over the others of being 1P instead of 2P, which approximately halves the available cooling and correspondingly limits clock speeds.

This doesn't make sense to me. I'd assume each system to be spec'ed correctly for the TDP of the processors. Each system has it's own HSF and case solution.

Multiprocessor systems do perform as less than 2x of their single processor counterparts, but it nothing to do with thermals in a properly designed system.

POWER doesn't have that many more threads. The top-of-the-line is 22-core 88-thread, compared to 32-core 64-thread from AMD and 28-core 56-thread from Intel.

I have no idea what having that many threads per core means for performance.

Offhand, I would guess that it's good for I/O-bound tasks, where you can have lots of threads waiting for input that don't need CPU time. A busy database maybe.

You don't need CPU threads for threads that don't need CPU...

The 8-core version only has 8 (each) ALUs, LSUs, and vector units. https://en.wikipedia.org/wiki/POWER9#Core If each core has 4 threads "running" on it, some of them are not going to be executing.

Still: the kernel won’t schedule threads that are waiting on I/O i believe.

Ok that's an interesting point. So they'd have to be waiting on a fetch instruction for the time not to be totally wasted?

Blocked tasks are not scheduled until unblocked.

Extra threads means that there's more likely to be something else available to schedule when one thread's pipelines are stalled due to a fetch from DRAM or MMIO, basically. There's very much a diminishing returns beyond two, but for some workloads (big mostly-RAM-resident data sets with poor locality of reference -- consumer databases say) it's worth it. It's unclear to me that 4-way multhreading is going to help any of the benchmarks in this test.

In the specific case of POWER 8 and 9 the cores are seriously overprevisioned with execution resources and you really need at least 2 threads running in order to make full use of them.

Could be. But the point was more that there's a very thin regime between "waiting for DRAM latency too often" (where more threads can help) and "bound by DRAM bandwidth" (where they won't). The DRAM isn't nearly as parallel as the cores are and saturates really fast.

Knights Corner was another case where you normally needed multiple threads.

In that case it was even more serious. Like the PPUs of the Cell processors, each core runs one instruction every other cycle. If you have only one thread, you effectively have half the throughput.

All diminishing returns really depend on your workload. We normally would say that for caches, with no point having enormous caches on desktop and typical x86 server processors, but IBM's mainframe CPUs have tons of L3 and even more tons of L4 caches, as well as dedicated cores (in the form of secondary processors) for offloading all kinds of tasks from the main CPUs, each with its own cache architecture.

Some of the models even have 8-way SMT!

Is there anyway to price out a power9 itself? Just for comparison.

Do you mean just the processor? RaptorCS sells Processors + main board bundles.

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