linux-ext4 - Re: [PATCH 1/6] mbcache2: Reimplement mbcache

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Message-ID: <20151222132950.GC7266@quack.suse.cz>
Date:	Tue, 22 Dec 2015 14:29:50 +0100
From:	Jan Kara <jack@...e.cz>
To:	Andreas Grünbacher 
	<andreas.gruenbacher@...il.com>
Cc:	Jan Kara <jack@...e.cz>, Ted Tso <tytso@....edu>,
	linux-ext4@...r.kernel.org, Laurent GUERBY <laurent@...rby.net>,
	Andreas Dilger <adilger@...ger.ca>
Subject: Re: [PATCH 1/6] mbcache2: Reimplement mbcache

On Tue 22-12-15 14:16:28, Andreas Grünbacher wrote:
> 2015-12-22 14:07 GMT+01:00 Jan Kara <jack@...e.cz>:
> > On Tue 22-12-15 13:20:58, Andreas Grünbacher wrote:
> >> That test scenario probably isn't very realistic: xattrs are mostly
> >> initialized at or immediately after file create time; they rarely
> >> removed. Hash chains should shrink significantly for that scenario.
> >
> > Well, the refcount gets dropped when the file itself is deleted. And that
> > isn't that rare. So I agree my benchmark isn't completely realistic in
> > changing the xattr but dropping xattr block refcount due to deleted file
> > will be relatively frequent so I don't thing refcount distribution obtained
> > by my benchmark is completely out.
> 
> Good point.
> 
> >> In addition, if the hash table is sized reasonably, long hash chains
> >> won't hurt that much because we can stop searching them as soon as we
> >> find the first reusable block. This won't help when there are hash
> >> conflicts, but those should be unlikely.
> >
> > Well, this is what happens currently as well...
> 
> Yes, except that many "unshareable" xattr blocks can remain in the
> hash table where they can only be skipped over. Since you have already
> written the code for fixing that and that fix won't make things worse,
> I would like to see that go in.

So for reference I'm attaching the patch which stops caching xattr block
when it reaches max refcount. Frankly, I'm not in favor of merging this
patch unless we can show it really improves noticeably some realistic
scenario. Removal and re-adding of xattr block from / to cache costs some
CPU cycles as well so overall I'm not convinced it is a clear win and
although the patch is simple it still adds some complexity.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

View attachment "0001-mbcache2-Do-not-cache-blocks-with-maximum-refcount.patch" of type "text/x-patch" (3803 bytes)