Workstation SKX is mis-identified

[loveshack]

I have a desktop SKX, model W-2123, which the cpuid code identifies as haswell (obvious with configure auto).

It turns out that it doesn't report avx512 vpus due to not parsing the model name. I fixed it with #351.

[fgvanzee]

@loveshack Thanks for your contribution, Dave. I was not even aware of this W line of parts. (I was confused at first because "desktop SKX" seemed contradictory; up until now we knew all desktop Skylakes to be sans AVX-512. But it seems the W is for workstation, which makes sense in that it's targeted at configurations that want AVX-512 but don't necessarily have space for the server-grade part, and/or don't need as many cores.)

[fgvanzee]

@devinamatthews Could you give your stamp of approval to this patch? @dnparikh seems to recall there being an issue of 1 VPU vs 2 VPUs, but I don't have any memory of this one way or another.

[devinamatthews]

Comments on #351

[devinamatthews]

up until now we knew all desktop Skylakes to be sans AVX-512

Except for Skylake-X i7 and i9! And Cannon Lake! And Cascade Lake!

[fgvanzee]

Except for Skylake-X i7 and i9! And Cannon Lake! And Cascade Lake!

Ugh. I know nothing, then.

Thanks for your comments, Devin.

[mratsim]

My Skylake-X i9-9980XE is also misidentified as Haswell.

[devinamatthews]

@mratsim can you send the output of /proc/cpuinfo or the equivalent on your platform?

[mratsim]

Here you go:

processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 85
model name	: Intel(R) Core(TM) i9-9980XE CPU @ 3.00GHz
stepping	: 4
microcode	: 0x2000043
cpu MHz		: 1406.852
cache size	: 25344 KB
physical id	: 0
siblings	: 36
core id		: 0
cpu cores	: 18
apicid		: 0
initial apicid	: 0
fpu		: yes
fpu_exception	: yes
cpuid level	: 22
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l3 cdp_l3 invpcid_single pti mba ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm cqm mpx rdt_a avx512f avx512dq rdseed adx smap clflushopt clwb intel_pt avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local dtherm ida arat pln pts
bugs		: cpu_meltdown spectre_v1 spectre_v2 spec_store_bypass l1tf mds swapgs taa itlb_multihit
bogomips	: 6002.00
clflush size	: 64
cache_alignment	: 64
address sizes	: 46 bits physical, 48 bits virtual
power management:

The full output is at https://gist.github.com/mratsim/419062e11ee1f66daa62c7fe4c13dc5d

[mratsim]

If that helps, for my own BLAS implementation purposes (see pytorch/pytorch#26534 (comment)), I only test if the CPU implements AVX512F (with CPUID instruction, leaf 7 -> ebx -> bit 16 is set), see my CPU detection code

[jeffhammond]

@mratsim Can you run https://github.com/jeffhammond/vpu-count to see if that detects AVX-512 2x FMA correctly?

[jeffhammond]

@fgvanzee @devinamatthews @loveshack I have to wonder if addressing my comment here (https://github.com/flame/blis/pull/351/files/94b34d38f6dffc074a4f12a1936c0ddba51f47ee..5597169dda1c4cb0c447cb589f8d5c2a5418a259#diff-81ef49aa7330af78381263dcf0acbea8) would solve this problem.

[jeffhammond]

@mratsim For context, the reason BLIS detects your CPU as Haswell is that it incorrectly thinks it has only 1 FMA unit. The handful of SKX processors that have only 1 FMA should be treated as Haswell by BLIS, for reasons discussed in #351.

[mratsim]

That's a good point, in my own code I assume that everyone who cared about numerical computing that got an AVX512 CPU was informed enough to get one with 2 FMA units.

Here is the output of the test.x script:

$  ./test.x 
0x0: 16,756e6547,6c65746e,49656e69
Intel? yes
0x1: 50654,16400800,7ffefbbf,bfebfbff
signature:  0x050654
model:      0x55=85
family:     0x06=6
ext model:  0x05=5
Skylake server? yes
0x7: 0,d39ffffb,0,c000000
Skylake AVX-512 detected
cpu_name = Intel(R) Core(TM) i9-9980XE CPU 
cpu_name[9] = C
cpu_name[17] =  
CPU has 2 AVX-512 VPUs

The empirical script however sometimes give me 1, sometimes give me 2, but my CPU is overclocked 4.1 GHz all-core turbo, 4.0 GHz all-core AVX turbo, 3.5 GHz all-core AVX512 turbo, see perf profile) so that overclocking + a non-performance CPU governor might throw off the script.

[loveshack]

My Skylake-X i9-9980XE is also misidentified as Haswell.

That should be fixed by the changes I submitted, which don't seem to be wanted. (If the CPU isn't recognized, the cpuid code might take a fraction of a second to run measurement code, but that needs a suitably-licensed implementation; Gromacs' has syntax for GCC, but is GPL. Otherwise, it's probably best just to default to the normal case of 2×FMA.) For what it's worth, it looks as if OpenBLAS now has decent (but unreleased) skylakex support, so if you just want a good BLAS, it will probably be the best option.

[jeffhammond]

@loveshack Your suggestion of OpenBLAS here is total garbage. Unless there has been a rewrite for SKX, it's nowhere near as fast on SKX. See SkylakeX on the Performance Wiki page for details.

Anyone can trivially fix the problem in this issue by setting the configuration name explicitly, which is how I just built a SKX binary on my HSW workstation. Using the build system options effectively is much easier than switch BLAS libraries.

./configure skx

I don't know what you are talking about with the licensing issues. vpu-count.c uses the MIT license. I am not going to ask for a license clarification on empirical.c because that method is worse in every way that matters to us. It is shown here once again giving wrong answers to @mratsim due to system noise.