From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries To: Michal Hocko , Dave Chinner Cc: James Bottomley , Linus Torvalds , Matthew Wilcox , Al Viro , Jonathan Corbet , "Luis R. Rodriguez" , Kees Cook , Linux Kernel Mailing List , linux-fsdevel , linux-mm , "open list:DOCUMENTATION" , Jan Kara , Paul McKenney , Andrew Morton , Ingo Molnar , Miklos Szeredi , Larry Woodman , "Wangkai (Kevin,C)" References: <1531330947.3260.13.camel@HansenPartnership.com> <18c5cbfe-403b-bb2b-1d11-19d324ec6234@redhat.com> <1531336913.3260.18.camel@HansenPartnership.com> <4d49a270-23c9-529f-f544-65508b6b53cc@redhat.com> <1531411494.18255.6.camel@HansenPartnership.com> <20180712164932.GA3475@bombadil.infradead.org> <1531416080.18255.8.camel@HansenPartnership.com> <1531425435.18255.17.camel@HansenPartnership.com> <20180713003614.GW2234@dastard> <20180716090901.GG17280@dhcp22.suse.cz> From: Waiman Long Message-ID: Date: Wed, 18 Jul 2018 12:17:24 -0400 MIME-Version: 1.0 In-Reply-To: <20180716090901.GG17280@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: On 07/16/2018 05:09 AM, Michal Hocko wrote: > On Fri 13-07-18 10:36:14, Dave Chinner wrote: > [...] >> By limiting the number of negative dentries in this case, internal >> slab fragmentation is reduced such that reclaim cost never gets out >> of control. While it appears to "fix" the symptoms, it doesn't >> address the underlying problem. It is a partial solution at best but >> at worst it's another opaque knob that nobody knows how or when to >> tune. > Would it help to put all the negative dentries into its own slab cache?= > >> Very few microbenchmarks expose this internal slab fragmentation >> problem because they either don't run long enough, don't create >> memory pressure, or don't have access patterns that mix long and >> short term slab objects together in a way that causes slab >> fragmentation. Run some cold cache directory traversals (git >> status?) at the same time you are creating negative dentries so you >> create pinned partial pages in the slab cache and see how the >> behaviour changes.... > Agreed! Slab fragmentation is a real problem we are seeing for quite > some time. We should try to address it rather than paper over it with > weird knobs. I am aware that you don't like the limit knob that control how many negative dentries are allowed as a percentage of total system memory. I got comments in the past about doing some kind of auto-tuning. How about consolidating the 2 knobs that I currently have in the patchset into a single one with 3 possible values, like: 0 - no limiting 1 - set soft limit to "a constant + 4 x max # of positive dentries" and warn if exceeded 2 - same limit but kill excess negative dentries after use. Does that kind of knob make more sense to you? Cheers, Longman