On Jun 10, 2010, at 4:57 AM, Sylvain Jeaugey wrote: > > As for why mmap is slower. When the file is on a real (not tmpfs or other > > ramdisk) I am 95% certain that this is an artifact of the Linux > > swapper/pager > > behavior which is thinking it is being smart by "swapping ahead". Even when > > there is no memory pressure that requires swapping, Linux starts queuing > > swap > > I/O for pages to keep the number of "clean" pages up when possible. This > > results in pages of the shared memory file being written out to the actual > > block device. Both the background I/O and the VM metadata updates > > contribute > > to the lost time. I say 95% certain because I have a colleague who looked > > into this phenomena in another setting and I am recounting what he reported > > as clearly as I can remember, but might have misunderstood or inserted my > > own > > speculation by accident. A sufficiently motivated investigator (not me) > > could probably devise an experiment to verify this. > Interesting. Do you think this behavior of the linux kernel would change > if the file was unlink()ed after attach ?
Note that OMPI does unlink the mmap'ed file after attach. -- Jeff Squyres jsquy...@cisco.com For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/
