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Date: Sat, 10 Nov 2001 15:56:03 +1100
From: Anton Blanchard <anton@samba.org>
To: Andi Kleen <ak@suse.de>
Cc: "David S. Miller" <davem@redhat.com>, mingo@elte.hu,
        linux-kernel@vger.kernel.org
Subject: Re: speed difference between using hard-linked and modular drives?
Message-ID: <20011110155603.B767@krispykreme>
In-Reply-To: <p731yj8kgvw.fsf@amdsim2.suse.de> <20011109110532.B6822@krispykreme> <20011109064540.A13498@wotan.suse.de> <20011108.220444.95062095.davem@redhat.com> <20011109073946.A19373@wotan.suse.de>
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Hi,

> I'm assuming that walking on average 5-10 pages on a lookup is not too big a 
> deal, especially when you use prefetch for the list walk. It is a tradeoff
> between a big hash table thrashing your cache and a smaller hash table that
> can be cached but has on average >1 entries/buckets. At some point the the 
> smaller hash table wins, assuming the hash function is evenly distributed.
> 
> It would only get bad if the average chain length would become much bigger.
> 
> Before jumping to real conclusions it would be interesting to gather
> some statistics on Anton's machine, but I suspect he just has an very
> unevenly populated table.

You can find the raw data here:

http://samba.org/~anton/linux/pagecache/pagecache_data_gfp.gz
http://samba.org/~anton/linux/pagecache/pagecache_data_vmalloc.gz

You can see the average depth of the get_free_page hash is way too deep.
I agree there are a lot of pagecache pages (17GB in the gfp test and 21GB
in the vmalloc test), but we have to make use of the 32GB of RAM :)

I did some experimentation with prefetch and I dont think it will gain
you anything here. We need to issue the prefetch many cycles before
using the data which we cannot do when walking the chain.

Anton
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