Messages in this thread | | | Date | Thu, 27 Nov 2008 11:14:36 +0100 | From | Nick Piggin <> | Subject | Re: [RFC v1][PATCH]page_fault retry with NOPAGE_RETRY |
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On Thu, Nov 27, 2008 at 11:00:07AM +0100, Peter Zijlstra wrote: > On Thu, 2008-11-27 at 01:28 -0800, Mike Waychison wrote: > > > Correct. I don't recall the numbers from the pathelogical cases we were > > seeing, but iirc, it was on the order of 10s of seconds, likely > > exascerbated by slower than usual disks. I've been digging through my > > inbox to find numbers without much success -- we've been using a variant > > of this patch since 2.6.11. > > > 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). > > If its only monitoring, there might be another solution. If you can keep > the required data in a separate (approximate) copy so that you don't > need mmap_sem at all to show them. > > If your mmap_sem is so contended your latencies are unacceptable, adding > more users to it - even statistics gathering, just isn't going to cure > the situation. > > Furthermore, /proc code usually isn't written with performance in mind, > so its usually simple and robust code. Adding it to a 'hot'-path like > you're doing doesn't seem advisable. > > Also, releasing and re-acquiring mmap_sem can significantly add to the > cacheline bouncing that thing already has.
Yes, it would be nice to reduce mmap_sem load regardless of any other fixes or problems. I guess they're not very worried about cacheline bouncing but more about hold time (how many sockets in these systems? 4 at most?)
I guess it is the pagemap stuff that they use most heavily?
pagemap_read looks like it can use get_user_pages_fast. The smaps and clear_refs stuff might have been nicer if they could work on ranges like pagemap. Then they could avoid mmap_sem as well (although maps would need to be sampled and take mmap_sem I guess).
One problem with dropping mmap_sem is that it hurts priority/fairness. And it opens a bit of a (maybe theoretical but not something to completely ignore) forward progress hole AFAIKS. If mmap_sem is very heavily contended, then the refault is going to take a while to get through, and then the page might get reclaimed etc).
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