Re: [RFC v1][PATCH]page_fault retry with NOPAGE_RETRY

[Nick Piggin]

On Thu, Nov 27, 2008 at 11:03:40AM -0800, Mike Waychison wrote:
> Nick Piggin wrote:
> >On Thu, Nov 27, 2008 at 01:28:41AM -0800, Mike Waychison wrote:
> >>
> >>Török however identified mmap taking on the order of several 
> >>milliseconds due to this exact problem:
> >>
> >>http://lkml.org/lkml/2008/9/12/185
> >
> >Turns out to be a different problem.
> >
> 
> What do you mean?

His is just contending on the write side. The retry patch doesn't help.


> >>We generally try to avoid such things, but sometimes it a) can't be 
> >>easily avoided (third party libraries for instance) and b) when it hits 
> >>us, it affects the overall health of the machine/cluster (the monitoring 
> >>daemons get blocked, which isn't very healthy).
> >
> >Are you doing appropriate posix_fadvise to prefetch in the files before
> >faulting, and madvise hints if appropriate?
> >
> 
> Yes, we've been slowly rolling out fadvise hints out, though not to 
> prefetch, and definitely not for faulting.  I don't see how issuing a 
> prefetch right before we try to fault in a page is going to help 
> matters.  The pages may appear in pagecache, but they won't be uptodate 
> by the time we look at them anyway, so we're back to square one.

The whole point of a prefetch is to issue it sufficiently early so
it makes a difference. Actually if you can tell quite well where the
major faults will be, but don't know it sufficiently in advance to
do very good prefetching, then perhaps we could add a new madvise hint
to synchronously bring the page in (dropping the mmap_sem over the IO).

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