[lkml]   [2015]   [Sep]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH 10/10] mm, page_alloc: Only enforce watermarks for order-0 allocations
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
> ( 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 <>
> Acked-by: Michal Hocko <>

Nice. This really is a great improvement over the way we used to
ensure higher-order page availability.

Acked-by: Johannes Weiner <>

 \ /
  Last update: 2015-09-25 21:41    [W:0.230 / U:1.068 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site