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Tasklist

FS#50688 - [linux] not all cpus detected on KNL (simple fix included)

Attached to Project: Arch Linux
Opened by Dimitar Pashov (dlp) - Wednesday, 07 September 2016, 07:41 GMT
Last edited by Tobias Powalowski (tpowa) - Friday, 09 September 2016, 07:46 GMT
Task Type Feature Request
Category Packages: Core
Status Closed
Assigned To Tobias Powalowski (tpowa)
Architecture x86_64
Severity Low
Priority Normal
Reported Version
Due in Version Undecided
Due Date Undecided
Percent Complete 100%
Votes 1
Private No

Details

Description: The kernel is configured to recognise no more than 128 cpus (hwthreads) which is at least half of what the new socketed Xeon Phi x200 (Knights Landing) offers. Would you please increase CONFIG_NR_CPUS to 288?


Additional info:
* linux-4.7+ and likely others
* I have compiled a kernel with CONFIG_NR_CPUS=256 and it seems to be working OK.
* KNL machines are single socket only so CONFIG_NR_CPUS does not need to go beyond 288 for the time being.


Steps to reproduce:
Boot a standard archlinux kernel on a KNL machine and notice how it only detects 128 cpus.
This task depends upon

Closed by  Tobias Powalowski (tpowa)
Friday, 09 September 2016, 07:46 GMT
Reason for closing:  Won't implement
Comment by Jan Alexander Steffens (heftig) - Thursday, 08 September 2016, 09:42 GMT
If you're running on Xeon Phi I think you'll want an optimized custom kernel anyway. A lot of stuff in the stock kernels is just useless overhead on a computing accelerator card.
Comment by AK (Andreaskem) - Thursday, 08 September 2016, 10:32 GMT
Knights Landing is no longer an accelerator card. It is its own full-blown processor that (supposedly) runs pretty much everything compiled for baseline x86-64 (with a lot of extensions for SIMD, etc. also available). I do not know why you would want to run Arch on such a beast, though.
Comment by Dimitar Pashov (dlp) - Friday, 09 September 2016, 05:53 GMT
Thanks for fixing the title.

Intel suggests running stock RHEL kernel and has released a temporary rpm for CentOS since it lags behind a bit. I've looked through most KNL specific patches and they are upstreamed so Arch kernel shall be OK as a start. When I have time I'll look into making a special package but it might well be more hassle than it is worth. On the benchmarks I've run so far I did not see any difference CentOS vs Arch. There are a few other patched packages (hwloc, memkind, mcelog) which I'll have to investigate. Hopefully their patches end up upstream too.

With that said, you'd want to run Arch 'on such a beast' for some of the reasons you'd want to run it on a desktop or laptop.
Comment by Tobias Powalowski (tpowa) - Friday, 09 September 2016, 07:36 GMT
I'm astonished on what CPU's people run arch on, the problem in bumping NR_CPU is it produces overhead for those who run smaller CPU's.
Each supported CPU needs memory and I don't know how apic and interrupts are affected on smaller systems by bumping it too high.
Better you compile your own kernel for now.

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