linux-kernel - Re: [PATCH 1/5] inode: Make unused inode LRU per superblock

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Date:	Thu, 27 May 2010 12:04:45 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, xfs@....sgi.com
Subject: Re: [PATCH 1/5] inode: Make unused inode LRU per superblock

On Thu, May 27, 2010 at 09:01:29AM +1000, Dave Chinner wrote:
> On Thu, May 27, 2010 at 02:17:33AM +1000, Nick Piggin wrote:
> > On Tue, May 25, 2010 at 06:53:04PM +1000, Dave Chinner wrote:
> > > From: Dave Chinner <dchinner@...hat.com>
> > > 
> > > The inode unused list is currently a global LRU. This does not match
> > > the other global filesystem cache - the dentry cache - which uses
> > > per-superblock LRU lists. Hence we have related filesystem object
> > > types using different LRU reclaimatin schemes.
> > 
> > Is this an improvement I wonder? The dcache is using per sb lists
> > because it specifically requires sb traversal.
> 
> Right - I originally implemented the per-sb dentry lists for
> scalability purposes. i.e. to avoid monopolising the dentry_lock
> during unmount looking for dentries on a specific sb and hanging the
> system for several minutes.
> 
> However, the reason for doing this to the inode cache is not for
> scalability, it's because we have a tight relationship between the
> dentry and inode cacheѕ. That is, reclaim from the dentry LRU grows
> the inode LRU.  Like the registration of the shrinkers, this is kind
> of an implicit, undocumented behavour of the current shrinker
> implemenation.

Right, that's why I wonder whether it is an improvement. It would
be interesting to see some tests (showing at least parity).

 
> What this patch series does is take that implicit relationship and
> make it explicit.  It also allows other filesystem caches to tie
> into the relationship if they need to (e.g. the XFS inode cache).
> What it _doesn't do_ is change the macro level behaviour of the
> shrinkers...
> 
> > What allocation/reclaim really wants (for good scalability and NUMA
> > characteristics) is per-zone lists for these things. It's easy to
> > convert a single list into per-zone lists.
> >
> > It is much harder to convert per-sb lists into per-sb x per-zone lists.
> 
> No it's not. Just convert the s_{dentry,inode}_lru lists on each
> superblock and call the shrinker with a new zone mask field to pick
> the correct LRU. That's no harder than converting a global LRU.
> Anyway, you'd still have to do per-sb x per-zone lists for the dentry LRUs,
> so changing the inode cache to per-sb makes no difference.

Right, it just makes it harder to do. By much harder, I did mostly mean
the extra memory overhead. If there is *no* benefit from doing per-sb
icache then I would question whether we should.

 
> However, this is a moot point because we don't have per-zone shrinker
> interfaces. That's an entirely separate discussion because of the
> macro-level behavioural changes it implies....

Yep. I have some patches for it, but they're currently behind the other
fine grained locking stuff. But it's something that really needs to be
implemented, IMO.

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