Re: [BUG] /proc/<pid>/stat access stalls badly for swapping process, 2.4.0-test10

[Val Henson <vhenson@esscom.com>]

On Thu, Nov 02, 2000 at 08:19:06AM +0100, Mike Galbraith wrote:
> On Wed, 1 Nov 2000, Rik van Riel wrote:
> 
> > I have one possible reason for this ....
> > 
> > 1) the procfs process does (in fs/proc/array.c::proc_pid_stat)
> > 	down(&mm->mmap_sem);
> > 
> > 2) but, in order to do that, it has to wait until the process
> >    it is trying to stat has /finished/ its page fault, and is
> >    not into its next one ...
> > 
> > 3) combine this with the elevator starvation stuff (ask Jens
> >    Axboe for blk-7 to alleviate this issue) and you have a
> >    scenario where processes using /proc/<pid>/stat have the
> >    possibility to block on multiple processes that are in the
> >    process of handling a page fault (but are being starved)
> 
> I'm experimenting with blk.[67] in test10 right now.  The stalls
> are not helped at all.  It doesn't seem to become request bound
> (haven't instrumented that yet to be sure) but the stalls persist.
> 
> 	-Mike

This is not an elevator starvation problem.

I also experienced these stalls with my IDE-only system.  Unless I'm
badly mistaken, the elevator is only used on SCSI disks, therefore
elevator starvation cannot be blamed for this problem.  These stalls
are particularly annoying since I want to find the pid of the process
hogging memory in order to kill it, but the read from /proc stalls for
45 seconds or more.

-VAL
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