From: Waiman Long <longman@redhat.com>
To: Michal Hocko <mhocko@kernel.org>, Dave Chinner <david@fromorbit.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Matthew Wilcox <willy@infradead.org>,
Al Viro <viro@zeniv.linux.org.uk>,
Jonathan Corbet <corbet@lwn.net>,
"Luis R. Rodriguez" <mcgrof@kernel.org>,
Kees Cook <keescook@chromium.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
Jan Kara <jack@suse.cz>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>,
Miklos Szeredi <mszeredi@redhat.com>,
Larry Woodman <lwoodman@redhat.com>,
"Wangkai (Kevin,C)" <wangkai86@huawei.com>
Subject: Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries
Date: Wed, 18 Jul 2018 12:17:24 -0400 [thread overview]
Message-ID: <caf309f5-2844-6546-e545-6ae56ad7d022@redhat.com> (raw)
In-Reply-To: <20180716090901.GG17280@dhcp22.suse.cz>
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
next prev parent reply other threads:[~2018-07-18 16:17 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-06 19:32 [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries Waiman Long
2018-07-06 19:32 ` [PATCH v6 1/7] fs/dcache: Track & report number " Waiman Long
2018-07-06 19:32 ` [PATCH v6 2/7] fs/dcache: Add sysctl parameter neg-dentry-pc as a soft limit on " Waiman Long
2018-07-06 19:32 ` [PATCH v6 3/7] fs/dcache: Enable automatic pruning of " Waiman Long
2018-07-06 19:32 ` [PATCH v6 4/7] fs/dcache: Spread negative dentry pruning across multiple CPUs Waiman Long
2018-07-06 19:32 ` [PATCH v6 5/7] fs/dcache: Add negative dentries to LRU head initially Waiman Long
2018-07-06 19:32 ` [PATCH v6 6/7] fs/dcache: Allow optional enforcement of negative dentry limit Waiman Long
2018-07-06 19:32 ` [PATCH v6 7/7] fs/dcache: Allow deconfiguration of negative dentry code to reduce kernel size Waiman Long
2018-07-06 21:54 ` Eric Biggers
2018-07-06 22:28 ` [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries Al Viro
2018-07-07 3:02 ` Waiman Long
2018-07-09 8:19 ` Michal Hocko
2018-07-09 16:01 ` Waiman Long
2018-07-10 14:27 ` Michal Hocko
2018-07-10 16:09 ` Waiman Long
2018-07-11 10:21 ` Michal Hocko
2018-07-11 15:13 ` Waiman Long
2018-07-11 17:42 ` James Bottomley
2018-07-11 19:07 ` Waiman Long
2018-07-11 19:21 ` James Bottomley
2018-07-12 15:54 ` Waiman Long
2018-07-12 16:04 ` James Bottomley
2018-07-12 16:26 ` Waiman Long
2018-07-12 17:33 ` James Bottomley
2018-07-13 15:32 ` Waiman Long
2018-07-12 16:49 ` Matthew Wilcox
2018-07-12 17:21 ` James Bottomley
2018-07-12 18:06 ` Linus Torvalds
2018-07-12 19:57 ` James Bottomley
2018-07-13 0:36 ` Dave Chinner
2018-07-13 15:46 ` James Bottomley
2018-07-13 23:17 ` Dave Chinner
2018-07-16 9:10 ` Michal Hocko
2018-07-16 14:42 ` James Bottomley
2018-07-16 9:09 ` Michal Hocko
2018-07-16 9:12 ` Michal Hocko
2018-07-16 12:41 ` Matthew Wilcox
2018-07-16 23:40 ` Andrew Morton
2018-07-17 1:30 ` Matthew Wilcox
2018-07-17 8:33 ` Michal Hocko
2018-07-19 0:33 ` Dave Chinner
2018-07-19 8:45 ` Michal Hocko
2018-07-19 9:13 ` Jan Kara
2018-07-18 18:39 ` Waiman Long
2018-07-18 16:17 ` Waiman Long [this message]
2018-07-19 8:48 ` Michal Hocko
2018-07-12 8:48 ` Michal Hocko
2018-07-12 16:12 ` Waiman Long
2018-07-12 23:16 ` Andrew Morton
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