Memory Subsystem: Bandwidth

Measuring the full bandwidth potential with John McCalpin's Stream bandwidth benchmark is getting increasingly difficult on the latest CPUs, as core and memory channel counts have continued to grow.  We compiled the stream 5.10 source code with the Intel compiler (icc) for linux version 17, or GCC 5.4, both 64-bit. The following compiler switches were used on icc:

icc -fast  -qopenmp  -parallel (-AVX) -DSTREAM_ARRAY_SIZE=800000000 

Notice that we had to increase the array significantly, to a data size of around 6 GB. We compiled one version with AVX and one without. 

The results are expressed in gigabytes per second.

Meanwhile the following compiler switches were used on gcc:

-Ofast -fopenmp -static -DSTREAM_ARRAY_SIZE=800000000

Notice that the DDR4 DRAM in the EPYC system ran at 2400 GT/s (8 channels), while the Intel system ran its DRAM at 2666 GT/s (6 channels). So the dual socket AMD system should theoretically get 307 GB per second (2.4 GT/s* 8 bytes per channel x 8 channels x 2 sockets). The Intel system has access to 256 GB per second (2.66 GT/s* 8 bytes per channel x 6 channels x 2 sockets).

Stream Triad (6 GB)

AMD told me they do not fully trust the results from the binaries compiled with ICC (and who can blame them?). Their own fully customized stream binary achieved 250 GB/s. Intel claims 199 GB/s for an AVX-512 optimized binary (Xeon E5-2699 v4: 128 GB/s with DDR-2400). Those kind of bandwidth numbers are only available to specially tuned AVX HPC binaries. 

Our numbers are much more realistic, and show that given enough threads, the 8 channels of DDR4 give the AMD EPYC server a 25% to 45% bandwidth advantage. This is less relevant in most server applications, but a nice bonus in many sparse matrix HPC applications. 

Maximum bandwidth is one thing, but that bandwidth must be available as soon as possible. To better understand the memory subsystem, we pinned the stream threads to different cores with numactl. 

Pinned Memory Bandwidth (in MB/sec)
Mem
Hierarchy
AMD "Naples"
EPYC 7601
DDR4-2400
Intel "Skylake-SP"
Xeon 8176
DDR4-2666
Intel "Broadwell-EP"
Xeon E5-2699v4
DDR4-2400
1 Thread 27490 12224 18555
2 Threads, same core
same socket
27663 14313 19043
2 Threads, different cores
same socket
29836 24462 37279
2 Threads, different socket 54997 24387 37333
4 threads on the first 4 cores
same socket
29201 47986 53983
8 threads on the first 8 cores
same socket
32703 77884 61450
8 threads on different dies 
(core 0,4,8,12...)
same socket
98747 77880 61504

The new Skylake-SP offers mediocre bandwidth to a single thread: only 12 GB/s is available despite the use of fast DDR-4 2666. The Broadwell-EP delivers 50% more bandwidth with slower DDR4-2400. It is clear that Skylake-SP needs more threads to get the most of its available memory bandwidth.

Meanwhile a single thread on a Naples core can get 27,5 GB/s if necessary. This is very promissing, as this means that a single-threaded phase in an HPC application will get abundant bandwidth and run as fast as possible. But the total bandwidth that one whole quad core CCX can command is only 30 GB/s.

Overall, memory bandwidth on Intel's Skylake-SP Xeon behaves more linearly than on AMD's EPYC. All off the Xeon's cores have access to all the memory channels, so bandwidth more directly increases with the number of threads. 

Testing Notes & Benchmark Configuration Memory Subsystem: Latency
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  • ddriver - Wednesday, July 12, 2017 - link

    LOL, buthurt intel fanboy claims that the only unbiased benchmark in the review is THE MOST biased benchmark in the review, the one that was done entirely for the puprpose to help intel save face.

    Because if many core servers running 128 gigs of ram are primarily used to run 16 megabyte databases in the real world. That's right!
    Reply
  • Beany2013 - Tuesday, July 11, 2017 - link

    Sure, test against Ubuntu 17.04 if you only plan to have your server running till January. When it goes end of life. That's not a joke - non LTS Ubuntu released get nine months patches and that's it.

    https://wiki.ubuntu.com/Releases

    16.04 is supported till 2021, it's what will be used in production by people who actually *buy* and *use* servers and as such it's a perfectly representative benchmark for people like me who are looking at dropping six figures on this level of hardware soon and want to see how it performs on...goodness, realistic workloads.
    Reply
  • rahvin - Wednesday, July 12, 2017 - link

    This is a silly argument. No one running these is going to be running bleeding edge software, compiling special kernels or putting optimizing compiler flags on anything. Enterprise runs on stable verified software and OS's. Your typical Enterprise Linux install is similar to RHEL 6 or 7 or it's variants (some are still running RHEL 5 with a 2.6 kernel!). Both RHEL6 and 7 have kernels that are 5+ years old and if you go with 6 it's closer to 10 year old.

    Enterprises don't run bleeding edge software or compile with aggressive flags, these things create regressions and difficult to trace bugs that cost time and lots of money. Your average enterprise is going to care about one thing, that's performance/watt running something like a LAMP stack or database on a standard vanilla distribution like RHEL. Any large enterprise is going to take a review like this and use it as data point when they buy a server and put a standard image on it and test their own workloads perf/watt.

    Some of the enterprises who are more fault tolerant might run something as bleeding edge as an Ubuntu Server LTS release. This review is a fair review for the expected audience, yes every writer has a little bias but I'd dare you to find it in this article, because the fanboi's on both sides are complaining that indicates how fair the review is.
    Reply
  • jjj - Tuesday, July 11, 2017 - link

    Do remember that the future is chiplets, even for Intel.
    The 2 are approaching that a bit differently as AMD had more cost constrains so they went with a 4 cores CCX that can be reused in many different prods.

    Highly doubt that AMD ever goes back to a very large die and it's not like Intel could do a monolithic 48 cores on 10nm this year or even next year and that would be even harder in a competitive market. Sure if they had a Cortex A75 like core and a lot less cache, that's another matter but they are so far behind in perf/mm2 that it's hard to even imagine that they can ever be that efficient.
    Reply
  • coder543 - Tuesday, July 11, 2017 - link

    Never heard the term "chiplet" before. I think AMD has adequately demonstrated the advantages (much higher yield -> lower cost, more than adequate performance), but I haven't heard Intel ever announce that they're planning to do this approach. After the embarrassment that they're experiencing now, maybe they will. Reply
  • Ian Cutress - Tuesday, July 11, 2017 - link

    Look up Intel's EMIB. It's an obvious future for that route to take as process nodes get smaller. Reply
  • Threska - Saturday, July 22, 2017 - link

    We may see their interposer (like used with their GPUs) technology being used. Reply
  • jeffsci - Tuesday, July 11, 2017 - link

    Benchmarking NAMD with pre-compiled binaries is pretty silly. If you can't figure out how to compile it for each every processor of interest, you shouldn't be benchmarking it. Reply
  • CajunArson - Tuesday, July 11, 2017 - link

    On top of all that, they couldn't even be bothered to download and install a (completely free) vanilla version that was released this year. Their version of NAMD 2.10 is from *2014*!

    http://www.ks.uiuc.edu/Development/Download/downlo...
    Reply
  • tamalero - Tuesday, July 11, 2017 - link

    Do high level servers update their versions constantly?
    I know that most of the critical stuff, only patch serious vulnerabilities and not update constantly to newer things just because they are available.
    Reply

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