Messages in this thread | | | Date | Tue, 09 Mar 2010 16:42:50 +0100 | From | Christian Ehrhardt <> | Subject | Re: [PATCH 1/3] page-allocator: Under memory pressure, wait on pressure to relieve instead of congestion |
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Nick Piggin wrote: > On Tue, Mar 09, 2010 at 02:17:13PM +0000, Mel Gorman wrote: >> On Wed, Mar 10, 2010 at 12:35:13AM +1100, Nick Piggin wrote: >>> On Mon, Mar 08, 2010 at 11:48:21AM +0000, Mel Gorman wrote: >>>> Under heavy memory pressure, the page allocator may call congestion_wait() >>>> to wait for IO congestion to clear or a timeout. This is not as sensible >>>> a choice as it first appears. There is no guarantee that BLK_RW_ASYNC is >>>> even congested as the pressure could have been due to a large number of >>>> SYNC reads and the allocator waits for the entire timeout, possibly uselessly. >>>> >>>> At the point of congestion_wait(), the allocator is struggling to get the >>>> pages it needs and it should back off. This patch puts the allocator to sleep >>>> on a zone->pressure_wq for either a timeout or until a direct reclaimer or >>>> kswapd brings the zone over the low watermark, whichever happens first. >>>> >>>> Signed-off-by: Mel Gorman <mel@csn.ul.ie> >>>> --- >>>> include/linux/mmzone.h | 3 ++ >>>> mm/internal.h | 4 +++ >>>> mm/mmzone.c | 47 +++++++++++++++++++++++++++++++++++++++++++++ >>>> mm/page_alloc.c | 50 +++++++++++++++++++++++++++++++++++++++++++---- >>>> mm/vmscan.c | 2 + >>>> 5 files changed, 101 insertions(+), 5 deletions(-) >>>> >>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h >>>> index 30fe668..72465c1 100644 >>>> --- a/include/linux/mmzone.h >>>> +++ b/include/linux/mmzone.h [...] >>>> +{ >>>> + /* If no process is waiting, nothing to do */ >>>> + if (!waitqueue_active(zone->pressure_wq)) >>>> + return; >>>> + >>>> + /* Check if the high watermark is ok for order 0 */ >>>> + if (zone_watermark_ok(zone, 0, low_wmark_pages(zone), 0, 0)) >>>> + wake_up_interruptible(zone->pressure_wq); >>>> +} >>> If you were to do this under the zone lock (in your subsequent patch), >>> then it could avoid races. I would suggest doing it all as a single >>> patch and not doing the pressure checks in reclaim at all. >>> >> That is reasonable. I've already dropped the checks in reclaim because as you >> say, if the free path check is cheap enough, it's also sufficient. Checking >> in the reclaim paths as well is redundant. >> >> I'll move the call to check_zone_pressure() within the zone lock to avoid >> races. >>
Mel, we talked about a thundering herd issue that might come up here in very constraint cases. So wherever you end up putting that wake_up call, how about being extra paranoid about a thundering herd flagging them WQ_FLAG_EXCLUSIVE and waking them with something like that:
wake_up_interruptible_nr(zone->pressure_wq, #nrofpagesabovewatermark#);
That should be an easy to calculate sane max of waiters to wake up. On the other hand it might be over-engineered and it implies the need to reconsider when it would be best to wake up the rest.
Get me right - I don't really have a hard requirement or need for that, I just wanted to mention it early on to hear your opinions about it.
looking forward to test the v2 patch series, adapted to all the good stuff already discussed.
--
Grüsse / regards, Christian Ehrhardt IBM Linux Technology Center, System z Linux Performance -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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