Messages in this thread | | | Date | Fri, 25 Sep 2015 15:32:06 -0400 | From | Johannes Weiner <> | Subject | Re: [PATCH 10/10] mm, page_alloc: Only enforce watermarks for order-0 allocations |
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On Mon, Sep 21, 2015 at 01:03:17PM +0100, Mel Gorman wrote: > The primary purpose of watermarks is to ensure that reclaim can always > make forward progress in PF_MEMALLOC context (kswapd and direct reclaim). > These assume that order-0 allocations are all that is necessary for > forward progress. > > High-order watermarks serve a different purpose. Kswapd > had no high-order awareness before they were introduced > (https://lkml.kernel.org/r/413AA7B2.4000907@yahoo.com.au). This was > particularly important when there were high-order atomic requests. > The watermarks both gave kswapd awareness and made a reserve for those > atomic requests. > > There are two important side-effects of this. The most important is that > a non-atomic high-order request can fail even though free pages are available > and the order-0 watermarks are ok. The second is that high-order watermark > checks are expensive as the free list counts up to the requested order must > be examined. > > With the introduction of MIGRATE_HIGHATOMIC it is no longer necessary to > have high-order watermarks. Kswapd and compaction still need high-order > awareness which is handled by checking that at least one suitable high-order > page is free. > > With the patch applied, there was little difference in the allocation > failure rates as the atomic reserves are small relative to the number of > allocation attempts. The expected impact is that there will never be an > allocation failure report that shows suitable pages on the free lists. > > The one potential side-effect of this is that in a vanilla kernel, the > watermark checks may have kept a free page for an atomic allocation. Now, > we are 100% relying on the HighAtomic reserves and an early allocation to > have allocated them. If the first high-order atomic allocation is after > the system is already heavily fragmented then it'll fail. > > Signed-off-by: Mel Gorman <mgorman@techsingularity.net> > Acked-by: Michal Hocko <mhocko@suse.com>
Nice. This really is a great improvement over the way we used to ensure higher-order page availability.
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
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