linux-kernel - Re: [PATCH v5 0/6] fs/dcache: Track & limit # of negative dentries

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Message-ID: <20180714173516.uumlhs4wgfgrlc32@devuan>
Date:   Sat, 14 Jul 2018 19:35:16 +0200
From:   Pavel Machek <pavel@....cz>
To:     Jan Kara <jack@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Waiman Long <longman@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Larry Woodman <lwoodman@...hat.com>,
        "Wangkai (Kevin,C)" <wangkai86@...wei.com>, linux-mm@...ck.org,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v5 0/6] fs/dcache: Track & limit # of negative dentries

> > Yes, "should be".  I could understand that the presence of huge
> > nunmbers of -ve dentries could result in undesirable reclaim of
> > pagecache, etc.  Triggering oom-killings is very bad, and presumably
> > has the same cause.
> > 
> > Before we go and add a large amount of code to do the shrinker's job
> > for it, we should get a full understanding of what's going wrong.  Is
> > it because the dentry_lru had a mixture of +ve and -ve dentries? 
> > Should we have a separate LRU for -ve dentries?  Are we appropriately
> > aging the various dentries?  etc.
> > 
> > It could be that tuning/fixing the current code will fix whatever
> > problems inspired this patchset.
> 
> What I think is contributing to the problems and could lead to reclaim
> oddities is the internal fragmentation of dentry slab cache. Dentries are
> relatively small, you get 21 per page on my system, so if trivial to
> reclaim negative dentries get mixed with a small amount of unreclaimable
> positive dentries, you can get a lot of pages in dentry slab cache that are
> unreclaimable.

Could we allocate -ve entries from separate slab?

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html