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Date: Tue, 10 Aug 2021 11:37:15 -0400 From: Waiman Long <llong@...hat.com> To: Nico Pache <npache@...hat.com>, linux-mm@...ck.org, akpm@...ux-foundation.org, linux-kernel@...r.kernel.org Cc: hannes@...xchg.org, aquini@...hat.com, shakeelb@...gle.com, llong@...hat.com, mhocko@...e.com, hakavlad@...ox.lv Subject: Re: [PATCH v3] vm_swappiness=0 should still try to avoid swapping anon memory On 8/9/21 6:37 PM, Nico Pache wrote: > Since commit 170b04b7ae49 ("mm/workingset: prepare the workingset detection > infrastructure for anon LRU") and commit b91ac374346b ("mm: vmscan: enforce > inactive:active ratio at the reclaim root") swappiness can start prematurely > swapping anon memory. This is due to the assumption that refaulting anon should > always allow the shrinker to target anon memory. Add a check for swappiness > being >0 before indiscriminately targeting Anon. Before these commits > when a user had swappiness=0 anon memory would rarely get swapped; this > behavior has remained constant sense RHEL5. This commit keeps that behavior Typo: "sense" -> "since" > intact and prevents the new workingset refaulting from challenging the anon > memory when swappiness=0. > > Anon can still be swapped to prevent OOM. This does not completely disable > swapping, but rather tames the refaulting aspect of the code that allows for > the deactivating of anon memory. > > We have two customer workloads that discovered this issue: > 1) A VM claiming 95% of the hosts memory followed by file reads (never dirty) > which begins to challenge the anon. Refaulting the anon working set will then > cause the indiscriminant swapping of the anon. > > 2) A VM running a in-memory DB is being populated from file reads. > Swappiness is set to 0 or 1 to defer write I/O as much as possible. Once > the customer experienced low memory, swapping anon starts, with > little-to-no PageCache being swapped. Pagecache are not swapped. It is discarded under memory pressure and written back if dirty. Other than that, the patch looks good to me. Cheers, Longman
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