linux-kernel - Re: [PATCH 2.6.19-rc3] VFS: per-sb dentry lru list

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Date:	Tue, 14 Nov 2006 09:12:45 +0300
From:	Vasily Averin <vvs@...ru>
To:	Neil Brown <neilb@...e.de>
CC:	David Howells <dhowells@...hat.com>, Kirill Korotaev <dev@...ru>,
	devel@...nvz.org, Andrew Morton <akpm@...l.org>,
	Kirill Korotaev <dev@...nvz.org>,
	Balbir Singh <balbir@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Olaf Hering <olh@...e.de>, Jan Blunck <jblunck@...e.de>
Subject: Re: [PATCH 2.6.19-rc3] VFS: per-sb dentry lru list

Hello Neil

Neil Brown wrote:
> On Wednesday November 1, vvs@...ru wrote:
>> Currently we have 3 type of functions that works with dentry_unused list:
>>
>> 1) prune_dcache(NULL) -- called from shrink_dcache_memory, frees the memory and
>> requires global LRU. works well in current implementation.
>> 2) prune_dcache(sb)  -- called from shrink_dcache_parent(), frees subtree, LRU
>> is not need here.  Current implementation uses global LRU for these purposes, it
>> is ineffective, and patch from Neil Brown fixes this issue.
>> 3) shrink_dcache_sb() -- called when we need to free the unused dentries for
>> given super block. Current implementation is not effective too, and per-sb LRU
>> would be the best solution here. On the other hand patch from Neil Brown is much
>> better than current implementation.
>>
>> In general I think that we should approve Neil Brown's patch. We (I and Kirill
>> Korotaev) are ready to acknowledge it when the following remarks fill be fixed:
> 
> 
>> - it seems for me list_splice() is not required inside
>>   prune_dcache(),
> 
> Yes, the list should be empty when we finish so you are right.
> 
>> - DCACHE_REFERENCED dentries should not be removed from private list to
>> dentry_unused list, this flag should be ignored if the private list is used,
> 
> Agreed.
> 
>> - count argument should be ignored in this case too, we want to free all the
>> dentries in private list,
> 
> Agreed.
> 
>> - when we shrink the whole super block we should free per-sb anonymous dentries
>> too (please see Kirill Korotaev's letter)
>>
> 
> Yes.  Unfortunately I don't think it is as easy as it sounds.
> I'll have a closer look.
> 
> 
>> Then I'm going to prepare new patch that will enhance the shrink_dcache_sb()
>> performance:
>> - we can add new list head into struct superblock and use it in
>> shrink_dcache_sb() instead of temporal private list. We will check is it empty
>> in dput() and add the new unused dentries to per-sb list instead of
>> dentry_unused list.
> 
> I think that makes sense.  It means that you end up doing less work in
> select_parent, because the work has already been done in dput.
> 
> How is the patch going?

Please see patches in attach files:
first one is incremental for your patch,
second one is merged version.

It would be very interesting to know your opinion about these changes.

thank you,
	Vasily Averin

View attachment "diff-vfs-shrink-dcache-NBaddon-20061103" of type "text/plain" (6385 bytes)

View attachment "diff-vfs-shrink-dcache-20061103" of type "text/plain" (8825 bytes)