From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933137AbbIYTcV (ORCPT ); Fri, 25 Sep 2015 15:32:21 -0400 Received: from gum.cmpxchg.org ([85.214.110.215]:47152 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932924AbbIYTcT (ORCPT ); Fri, 25 Sep 2015 15:32:19 -0400 Date: Fri, 25 Sep 2015 15:32:06 -0400 From: Johannes Weiner To: Mel Gorman Cc: Andrew Morton , Rik van Riel , Vlastimil Babka , David Rientjes , Joonsoo Kim , Michal Hocko , Linux-MM , LKML Subject: Re: [PATCH 10/10] mm, page_alloc: Only enforce watermarks for order-0 allocations Message-ID: <20150925193206.GE16359@cmpxchg.org> References: <1442832762-7247-1-git-send-email-mgorman@techsingularity.net> <20150921120317.GC3068@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150921120317.GC3068@techsingularity.net> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > 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