Re: How can I optimize a process on a NUMA architecture(x86-64 specifically)?

["Brett E." <brettspamacct@fastclick.com>]

Martin J. Bligh wrote:

>>Say you have a bunch of single-threaded processes on a NUMA machine. 
>>Does the kernel make sure to prefer allocations using a certain CPU's 
>>memory, preferring to run a given process on the CPU which contains 
>>its memory?  Or should I use the NUMA API(libnuma) to spell this out 
>>to the kernel? Does the kernel do the right thing in this case?
> 
> 
> The kernel will generally do the right thing (process local alloc) by
> default. In 99% of cases, you don't want to muck with it - unless you're
> running one single app dominating the whole system, and nothing else is
> going on, you probably don't want to specify anything explicitly.
> 
> M.
> 
Let's say I have a 2 way opteron and want to run 4 long-lived processes. 
  I fork and exec to create 1 of the processes, it chooses to run on 
processor 0 since processor 1 is overloaded at that time, so its 
homenode is processor 0.  I fork and exec another, it chooses processor 
0 since processors 1 is overloaded at that time. .. Let's say an uneven 
distribution is chosen for all 4 processes, with all processes mapped to 
processor 0. So they allocate on node 0 yet the scheduler will map these 
to both processors since CPU should be balanced. In this case, you will 
have a situation where the second processor will have to fetch memory 
from the other processor's memory.

So a better solution would be to use numactl to set the homenodes 
explicitly, choosing processor 0 for 2 processes, processor 1 for the 2 
other processes.

Is this incorrect?