John R Pierce <pierce@xxxxxxxxxxxx> writes:
> On 10/1/2017 8:38 AM, hw wrote:
>> HP says that what they call "NUMA split mode" should be disabled in the
>> BIOS of the Z800 workstation when running Linux. They are reasoning
>> that Linux kernels do not support this feature and even might not boot
>> if it´s enabled.
>
> hmm, that workstation is a dual Xeon 56xx (Westmere-EP, derived from
> Nehalem), new in 2010
>
>> Since it apparently was years ago since they made this statement, I´m
>> wondering if I should still leave this feature disabled or not. More
>> recent kernels might support it, and it´s supposed to improve
>> performance.
>>
>> Could someone explain what this feature actually is or does, and if
>> Centos kernels support it?
>
>
> On these sorts of dual socket hardware architectures, half of the
> memory is directly attached to each CPU, and the two CPUs are linked
> with a QPI bus. All the memory appears in one unified address space,
> but the memory belonging to the 'other' CPU has a little higher
> latency to access since it has to go across the QPI. In non-NUMA
> mode, this is ignored, and all memory is treated as equal from the OS
> perspective. in NUMA mode, an attempt is made to keep process memory
> on one CPU's memory, and to prefer scheduling those processes on the
> cores of that CPU. This can get messy, say you have a process running
> on core 0 (in cpu0) which allocates a big block of shared memory, then
> spawns 8 worker threads which all run concurrently and use this same
> shared working memory space. there's only 4 or 6 cores on each of
> the two CPUs, so either these worker threads have to wait for an
> available core on the same CPU as the memory allocation, or some of
> them end up running across the QPI bus anyways.
>
> I believe Linux, even RHEL 6, does support NUMA configurations, but
> its very questionable if a random typical workload would actually gain
> much from it, and it adds significant overhead in keeping track of all
> this.
Is it possible that you are confusing enabling/disabling NUMA with NUMA
split mode?
It is possible to disable/enable NUMA, and when NUMA is enabled, you can
also enable the mysterious NUMA split mode.
I´m trying to download the PDF you pointed me to, but the download is
stalled. I´m running Centos 7.4, but perhaps there´s an explanation
in the PDF that might tell me what NUMA split mode is supposed to be.
So far, I found out that KSM is disabled by default and would probably
be a disadvantage here, so I´m using numad and probably gain something
from most, if not all, things using local memory instead of going across
nodes. This will need some further investigation, though.
--
"Didn't work" is an error.
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