From: David Chinner <dgc@sgi.com>
Subject: Re: [PATCH 4/8] knfsd: repcache: split hash index
Date: Mon, 16 Oct 2006 16:38:58 +1000
Message-ID: <20061016063858.GI11034@melbourne.sgi.com>
References: <1160566044.8530.13.camel@hole.melbourne.sgi.com>
	<17714.59304.768727.298610@cse.unsw.edu.au>
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Cc: Linux NFS Mailing List <nfs@lists.sourceforge.net>,
	Greg Banks <gnb@melbourne.sgi.com>
To: Neil Brown <neilb@suse.de>
In-Reply-To: <17714.59304.768727.298610@cse.unsw.edu.au>
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On Mon, Oct 16, 2006 at 12:00:08PM +1000, Neil Brown wrote:
> If I've done my sums right (there is always room for doubt), then HASHSIZE == 4096.
> 
> > +
> > +		b->hash = kmalloc (HASHSIZE * sizeof(struct hlist_head), GFP_KERNEL);
> 
> So this kmalloc asks for 16K or 32K depending on pointer size.  On
> most machines that would be an order 2 or 3 allocation which is more
> likely to fail that order 0.
>
> I would really like to see HASHSIZE limited to PAGE_SIZE, and if
> needed, push CACHE_NUM_BUCKETS up ... that might make the
> 'cache_buckets' array bigger than a page, but we don't kmalloc that so
> it shouldn't be a problem.
> 
> Hmmm.. but if we wanted to scale the hash table size based on memory,
> we would want to kmalloc cache_buckets which might limit it's size...
> Maybe we could try allocating it big - which might work on big
> machines, and scale back the size on kmalloc failure??  That would
> probably work.

Yes, that works.  e.g. XFS uses kmem_zalloc_greedy() allocating
large hashes at mount time....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group

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