[Beowulf] Again about NUMA (numactl and taskset)

Håkon Bugge Hakon.Bugge at scali.com
Tue Jun 24 06:15:11 EDT 2008


At 07:56 24.06.2008, Chris Samuel wrote:
> > Your MPI (and OpenMP) should do this for you.
>
>Although not always correctly, it may assume that it can
>allocate from core 0 onwards leading to odd performance
>issues if you happen to get two 4 CPU jobs running on the
>same node..

If the MPI is doing that it is real stupid. 
Physical core IDs are physical ones. Users and 
applications should stay away from binding to 
physical resources. They can be used by others! 
These physical IDs varies greatly, and it has 
even been observed different enumeration on the 
same mothbd between boots. Imagine how to operate 
a cluster of these when users enumerate these IDs specifically ;-)

IMHO, the MPI should virtualize these resources 
and relieve the end-user/application programmer 
from the burden. Here's one example using Scali 
MPI Connect running two jobs, each with four MPI 
processes on a dual-socket, quad-core Barcelona system:

First job:
(submon-1 at barcelona-1) Affinity 'automatic' 
policy BANDWIDTH granularity CORE nprocs 4
(submon-1 at barcelona-1) Will bind process 0 with 
mask=0000000000000001 [(board=0, socket=0, core=0, execunit=0)]
(submon-1 at barcelona-1) Will bind process 1 with 
mask=0000000000010000 [(board=0, socket=1, core=0, execunit=0)]
(submon-1 at barcelona-1) Will bind process 2 with 
mask=0000000000000010 [(board=0, socket=0, core=1, execunit=0)]
(submon-1 at barcelona-1) Will bind process 3 with 
mask=0000000000100000 [(board=0, socket=1, core=1, execunit=0)]

Second job:
(submon-1 at barcelona-1) Affinity 'automatic' 
policy BANDWIDTH granularity CORE nprocs 4
(submon-1 at barcelona-1) Will bind process 0 with 
mask=0000000000000100 [(board=0, socket=0, core=2, execunit=0)]
(submon-1 at barcelona-1) Will bind process 1 with 
mask=0000000001000000 [(board=0, socket=1, core=2, execunit=0)]
(submon-1 at barcelona-1) Will bind process 2 with 
mask=0000000000001000 [(board=0, socket=0, core=3, execunit=0)]
(submon-1 at barcelona-1) Will bind process 3 with 
mask=0000000010000000 [(board=0, socket=1, core=3, execunit=0)]

and, this is with default settings. Other 
policies and 'resolutions' as we call it can be applied.


Håkon


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