If anyone is up for it, another interesting performance comparison could
be start-up time. That is, consider a fat node with many on-node
processes and a large shared-memory area. How long does it take for all
that shared memory to be set up? Arguably, start-up time is a
"second-order effect", but it ends up mattering to someone sometime. I
can't find my notes on this, but I thought SysV had some nice advantages
in this area, but there were many other complicating factors (even
beyond the clean-up stuff Jeff mentions).
Jeff Squyres wrote:
Iiinnnnteresting.
This, of course, begs the question of whether we should use sysv shmem or not.
It seems like the order of preference should be:
- sysv
- mmap in a tmpfs
- mmap in a "regular" (but not networked) fs
The big downer, of course, is the whole "what happens if the job crashes?"
issue. With mmap, an rm -rf will clean up any leftover files (although looking for them
in /dev/shm might be a bit non-obvious). With sysv, you have to use the ipc* commands to
look for and whack any orphan shmem segments.
Right now, the orted/hnp won't clean up any left over sysv segments. This
seems like something we should fix.
But even with that, if the orted/hnp is killed, sysv segments can get let over.
Hrm.
On Jun 9, 2010, at 11:58 AM, Sylvain Jeaugey wrote:
As stated at the conf call, I did some performance testing on a 32 cores
node.
So, here is graph showing 500 timings of an allreduce operation (repeated
15,000 times for good timing) with sysv, mmap on /dev/shm and mmap on
/tmp.
What is shows :
- sysv has the better performance ;
- having the mmap file in /dev/shm is very close to sysv. We only have
+0.1 us for a complete allreduce operation, but it seems stable. The noise
is identical to sysv (must be OS noise) ;
- having the mmap file in /tmp (ext3) decreases performance (+0.4 us
compared to /dev/shm) and seems prone to some "other" noise.
Warning : the graph does not start at 0.
On Tue, 27 Apr 2010, Samuel K. Gutierrez wrote:
With Jeff and Ralph's help, I have completed a System V shared memory
component for Open MPI. I have conducted some preliminary tests on our
systems, but would like to get test results from a broader audience.
As it stands, mmap is the defaul, but System V shared memory can be activated
using: -mca mpi_common_sm sysv
Repository:
http://bitbucket.org/samuelkgutierrez/ompi_sysv_sm
Input is greatly appreciated!
