[Beowulf] NUMA info request

Eric Thibodeau kyron at neuralbs.com
Tue Mar 25 00:17:04 EDT 2008


Mark Hahn wrote:
>>   NUMA is an acronym meaning Non Uniform Memory Access. This is a 
>> hardware constraint and is not a "performance" switch you turn on. 
>> Under the Linux
>
> I don't agree.  NUMA is indeed a description of hardware.  I'm not 
> sure what you meant by "constraint" - NUMA is not some kind of 
> shortcoming.
Mark is right, my choice of words is misleading. By constraint I meant 
that you have to be conscious of what ends up where (that was the point 
of the link I added in my e-mail ;P )
>> kernel there is an option that is meant to tell the kernel to be 
>> conscious about that hardware fact and attempt to help it optimize 
>> the way it maps the memory allocation to a task Vs the processor the 
>> given task will be using (processor affinity, check out taskset (in 
>> recent util-linux implementations, ie: 2.13+).
> the kernel has had various forms of NUMA and socket affinity for a 
> long time,
> and I suspect most any distro will install kernel which has the 
> appropriate support (surely any x86_64 kernel would have NUMA support).
My point of view on distro kernels is that they are to be scrutinized 
unless they are specifically meant to be used as computation nodes (ie: 
don't expect CONFIG_HZ=100 to be set on "typical" distros).
Also, NUMA is only applicable to Opteron architecture (internal MMU with 
HyperTransport), not the Intel flavor of multi-core CPUs (external MMU, 
which can be a single bus or any memory access scheme as dictated by the 
motherboard manufacturer).
>
> I usually use numactl rather than taskset.  I'm not sure of the 
> history of those tools.  as far as I can tell, taskset only addresses 
> numactl --cpubind,
> though they obviously approach things differently.  if you're going to 
> use taskset, you'll want to set cpu affinity to multiple cpus (those 
> local to a socket, or 'node' in numactl terms.)
>
>>   In your specific case, you would have 4Gigs per CPU and would want 
>> to make sure each task (assuming one per CPU) stays on the same CPU 
>> all the time and would want to make sure each task fits within the 
>> "local" 4Gig.
>
> "numactl --localalloc".
>
> but you should first verify that your machines actually do have the 8GB
> split across both nodes.  it's not that uncommon to see an 
> inexperienced assembler fill up one node before going onto the next, 
> and there have even
> been some boards which provided no memory to the second node.
Mark (Hahn) is right (again !), I ASSumed the tech would load the memory 
banks appropriately, don't make that mistake ;) And numactl is indeed 
more appropriate in this case (thanks Mr. Hahn ;) ). Note that the 
kernel (configured with NUMA) _will_ attempt to allocate the memory to 
"'local nodes" before offloading to memory "abroad".

Eric
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