From: Waiman Long <longman@redhat.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
Jonathan Corbet <corbet@lwn.net>,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-mm@kvack.org, linux-doc@vger.kernel.org,
"Luis R. Rodriguez" <mcgrof@kernel.org>,
Kees Cook <keescook@chromium.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Jan Kara <jack@suse.cz>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>,
Miklos Szeredi <mszeredi@redhat.com>,
Matthew Wilcox <willy@infradead.org>,
Larry Woodman <lwoodman@redhat.com>,
James Bottomley <James.Bottomley@HansenPartnership.com>,
"Wangkai (Kevin C)" <wangkai86@huawei.com>
Subject: Re: [PATCH 2/2] fs/dcache: Make negative dentries easier to be reclaimed
Date: Thu, 30 Aug 2018 17:48:50 -0400 [thread overview]
Message-ID: <9b5c3e96-9dcc-e601-9d15-116aef4bdbfb@redhat.com> (raw)
In-Reply-To: <20180830072056.GC2656@dhcp22.suse.cz>
On 08/30/2018 03:20 AM, Michal Hocko wrote:
> On Wed 29-08-18 15:58:52, Waiman Long wrote:
>> On 08/29/2018 03:51 AM, Michal Hocko wrote:
>>> On Tue 28-08-18 13:19:40, Waiman Long wrote:
>>>> For negative dentries that are accessed once and never used again, they
>>>> should be removed first before other dentries when shrinker is running.
>>>> This is done by putting negative dentries at the head of the LRU list
>>>> instead at the tail.
>>>>
>>>> A new DCACHE_NEW_NEGATIVE flag is now added to a negative dentry when it
>>>> is initially created. When such a dentry is added to the LRU, it will be
>>>> added to the head so that it will be the first to go when a shrinker is
>>>> running if it is never accessed again (DCACHE_REFERENCED bit not set).
>>>> The flag is cleared after the LRU list addition.
>>> Placing object to the head of the LRU list can be really tricky as Dave
>>> pointed out. I am not familiar with the dentry cache reclaim so my
>>> comparison below might not apply. Let me try anyway.
>>>
>>> Negative dentries sound very similar to MADV_FREE pages from the reclaim
>>> POV. They are primary candidate for reclaim, yet you want to preserve
>>> aging to other easily reclaimable objects (including other MADV_FREE
>>> pages). What we do for those pages is to move them from the anonymous
>>> LRU list to the inactive file LRU list. Now you obviously do not have
>>> anon/file LRUs but something similar to active/inactive LRU lists might
>>> be a reasonably good match. Have easily reclaimable dentries on the
>>> inactive list including negative dentries. If negative entries are
>>> heavily used then they can promote to the active list because there is
>>> no reason to reclaim them soon.
>>>
>>> Just my 2c
>> As mentioned in my reply to Dave, I did considered using a 2 LRU list
>> solution. However, that will add more complexity to the dcache LRU
>> management code than my current approach and probably more potential for
>> slowdown.
> I completely agree with Dave here. This is not easy but trying to sneak
> in something that works for an _artificial_ workload is simply a no go.
> So if it takes to come with a more complex solution to cover more
> general workloads then be it. Someone has to bite a bullet and explore
> that direction. It won't be a simple project but well, if negative
> dentries really matter then it is worth making the reclaim design robust
> and comprehensible rather than adhoc and unpredictable.
OK, I will need to spend more time to think about a better way of doing
that.
Cheers,
Longman
next prev parent reply other threads:[~2018-08-30 21:48 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-28 17:19 [PATCH 0/2] fs/dcache: Track # of negative dentries Waiman Long
2018-08-28 17:19 ` [PATCH 1/2] fs/dcache: Track & report number " Waiman Long
2018-08-29 0:11 ` Dave Chinner
2018-08-29 17:11 ` Waiman Long
2018-08-30 1:43 ` Dave Chinner
2018-08-30 21:49 ` Waiman Long
2018-08-31 14:31 ` Matthew Wilcox
2018-08-31 15:03 ` Waiman Long
2018-08-28 17:19 ` [PATCH 2/2] fs/dcache: Make negative dentries easier to be reclaimed Waiman Long
2018-08-28 22:13 ` Matthew Wilcox
2018-08-28 22:29 ` Waiman Long
2018-08-28 23:10 ` Linus Torvalds
2018-08-28 23:22 ` Andrew Morton
2018-08-29 1:18 ` Waiman Long
2018-08-29 1:18 ` Waiman Long
2018-08-28 23:01 ` Andrew Morton
2018-08-29 17:54 ` Paul E. McKenney
2018-08-29 20:03 ` Waiman Long
2018-08-29 21:04 ` Matthew Wilcox
2018-08-29 1:02 ` Dave Chinner
2018-08-29 19:34 ` Waiman Long
2018-08-30 1:12 ` Dave Chinner
2018-08-30 21:51 ` Waiman Long
2018-08-29 7:51 ` Michal Hocko
2018-08-29 19:58 ` Waiman Long
2018-08-30 7:20 ` Michal Hocko
2018-08-30 21:48 ` Waiman Long [this message]
2018-08-28 22:50 ` [PATCH 0/2] fs/dcache: Track # of negative dentries Andrew Morton
2018-08-28 22:54 ` Waiman Long
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=9b5c3e96-9dcc-e601-9d15-116aef4bdbfb@redhat.com \
--to=longman@redhat.com \
--cc=James.Bottomley@HansenPartnership.com \
--cc=akpm@linux-foundation.org \
--cc=corbet@lwn.net \
--cc=jack@suse.cz \
--cc=keescook@chromium.org \
--cc=linux-doc@vger.kernel.org \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=lwoodman@redhat.com \
--cc=mcgrof@kernel.org \
--cc=mhocko@kernel.org \
--cc=mingo@kernel.org \
--cc=mszeredi@redhat.com \
--cc=paulmck@linux.vnet.ibm.com \
--cc=torvalds@linux-foundation.org \
--cc=viro@zeniv.linux.org.uk \
--cc=wangkai86@huawei.com \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).