From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f199.google.com (mail-pf0-f199.google.com [209.85.192.199]) by kanga.kvack.org (Postfix) with ESMTP id A1495828E1 for ; Wed, 29 Jun 2016 16:55:57 -0400 (EDT) Received: by mail-pf0-f199.google.com with SMTP id e189so127542950pfa.2 for ; Wed, 29 Jun 2016 13:55:57 -0700 (PDT) Received: from mail-pf0-x22b.google.com (mail-pf0-x22b.google.com. [2607:f8b0:400e:c00::22b]) by mx.google.com with ESMTPS id o81si220900pfa.34.2016.06.29.13.55.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 29 Jun 2016 13:55:56 -0700 (PDT) Received: by mail-pf0-x22b.google.com with SMTP id t190so21644824pfb.3 for ; Wed, 29 Jun 2016 13:55:56 -0700 (PDT) Date: Wed, 29 Jun 2016 13:55:55 -0700 (PDT) From: David Rientjes Subject: Re: [patch] mm, compaction: make sure freeing scanner isn't persistently expensive In-Reply-To: <6685fe19-753d-7d76-aced-3bb071d7c81d@suse.cz> Message-ID: References: <6685fe19-753d-7d76-aced-3bb071d7c81d@suse.cz> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: Andrew Morton , Joonsoo Kim , Mel Gorman , linux-mm@kvack.org, linux-kernel@vger.kernel.org On Wed, 29 Jun 2016, Vlastimil Babka wrote: > On 06/29/2016 03:39 AM, David Rientjes wrote: > > It's possible that the freeing scanner can be consistently expensive if > > memory is well compacted toward the end of the zone with few free pages > > available in that area. > > > > If all zone memory is synchronously compacted, say with > > /proc/sys/vm/compact_memory, and thp is faulted, it is possible to > > iterate a massive amount of memory even with the per-zone cached free > > position. > > > > For example, after compacting all memory and faulting thp for heap, it > > was observed that compact_free_scanned increased as much as 892518911 4KB > > pages while compact_stall only increased by 171. The freeing scanner > > iterated ~20GB of memory for each compaction stall. > > > > To address this, if too much memory is spanned on the freeing scanner's > > freelist when releasing back to the system, return the low pfn rather than > > the high pfn. It's declared that the freeing scanner will become too > > expensive if the high pfn is used, so use the low pfn instead. > > > > The amount of memory declared as too expensive to iterate is subjectively > > chosen at COMPACT_CLUSTER_MAX << PAGE_SHIFT, which is 512MB with 4KB > > pages. > > > > Signed-off-by: David Rientjes > > Hmm, I don't know. Seems it only works around one corner case of a larger > issue. The cost for the scanning was already paid, the patch prevents it from > being paid again, but only until the scanners are reset. > The only point of the per-zone cached pfn positions is to avoid doing the same work again unnecessarily. Having the last 16GB of memory at the end of a zone being completely unfree is the same as a single page in the last pageblock free. The number of PageBuddy pages in that amount of memory can be irrelevant up to COMPACT_CLUSTER_MAX. We simply can't afford to scan 16GB of memory looking for free pages. > Note also that THP's no longer do direct compaction by default in recent > kernels. > > To fully solve the freepage scanning issue, we should probably pick and finish > one of the proposed reworks from Joonsoo or myself, or the approach that > replaces free scanner with direct freelist allocations. > Feel free to post the patches, but I believe this simple change makes release_freepages() exceedingly better and can better target memory for the freeing scanner. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org