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Date:   Wed, 11 Jul 2018 12:21:39 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Jonathan Corbet <corbet@lwn.net>,
        "Luis R. Rodriguez" <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-doc@vger.kernel.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 v6 0/7] fs/dcache: Track & limit # of negative dentries
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On Tue 10-07-18 12:09:17, Waiman Long wrote:
> On 07/10/2018 10:27 AM, Michal Hocko wrote:
> > On Mon 09-07-18 12:01:04, Waiman Long wrote:
> >> On 07/09/2018 04:19 AM, Michal Hocko wrote:
[...]
> >>> percentage has turned out to be a really wrong unit for many tunables
> >>> over time. Even 1% can be just too much on really large machines.
> >> Yes, that is true. Do you have any suggestion of what kind of unit
> >> should be used? I can scale down the unit to 0.1% of the system memory.
> >> Alternatively, one unit can be 10k/cpu thread, so a 20-thread system
> >> corresponds to 200k, etc.
> > I simply think this is a strange user interface. How much is a
> > reasonable number? How can any admin figure that out?
> 
> Without the optional enforcement, the limit is essentially just a
> notification mechanism where the system signals that there is something
> wrong going on and the system administrator need to take a look. So it
> is perfectly OK if the limit is sufficiently high that normally we won't
> need to use that many negative dentries. The goal is to prevent negative
> dentries from consuming a significant portion of the system memory.

So again. How do you tell the right number?

> I am going to reduce the granularity of each unit to 1/1000 of the total
> system memory so that for large system with TB of memory, a smaller
> amount of memory can be specified.

It is just a matter of time for this to be too coarse as well.
-- 
Michal Hocko
SUSE Labs