linux-ext4 - Re: [RFC PATCH v2 0/4] ext4: extents status tree shrinker improvement

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Date:	Tue, 22 Apr 2014 09:10:02 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Zheng Liu <gnehzuil.liu@...il.com>, linux-ext4@...r.kernel.org,
	Zheng Liu <wenqing.lz@...bao.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Jan Kara <jack@...e.cz>
Subject: Re: [RFC PATCH v2 0/4] ext4: extents status tree shrinker improvement

On Thu, Apr 17, 2014 at 11:35:26AM -0400, Theodore Ts'o wrote:
> So I've been thinking about this some more, and it seems to me is
> actually, what we need is *both* an LRU and a RR scheme.

We already have shrinker implementations that do this. It would
probably take 10-15 lines of code to add it to any existing LRU
list based shrinker.....

> The real problem here is that we have workloads that are generating a
> large number of "low value" extent cache entries.  That is, they are
> extremely unlikely to be used again, because they are small, and being
> generated when you have a highly fragmented extent status cache, and
> very often, the workload is a random read/write workload, so there is
> no way the full "working set" of extent cache entries could be kept in
> memory at the same time anyway.  These less valuable cache entries are
> being generated at a very high rate, and we want to make sure we don't
> penalize the "valuable" cache entries.

Yup, an "object referenced" bit that gets set on a cache lookup hit.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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