linux-kernel - Re: [RFC][PATCH] on-demand readahead

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Message-ID: <p73odlc34ga.fsf@bingen.suse.de>
Date:	25 Apr 2007 16:37:41 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Fengguang Wu <fengguang.wu@...il.com>
Cc:	Andrew Morton <akpm@...l.org>, Oleg Nesterov <oleg@...sign.ru>,
	Steven Pratt <slpratt@...tin.ibm.com>,
	Ram Pai <linuxram@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] on-demand readahead

Fengguang Wu <fengguang.wu@...il.com> writes:

> OVERHEADS
> 
> The new code reduced the overheads of
> 
> 	- excessively calling the readahead routine on small sized reads
> 	  (the current readahead code insists on seeing all requests)
> 
> 	- doing a lot of pointless page-cache lookups for small cached files
> 	  (the current readahead only turns itself off after 256 cache hits,
> 	  unfortunately most files are < 1MB, so never see that chance)

Would it make sense to keep track in the AS if the file is completely in cache?
Then you could probably avoid a lot of these lookups for small in cache files

> --- linux-2.6.21-rc7-mm1.orig/mm/readahead.c
> +++ linux-2.6.21-rc7-mm1/mm/readahead.c
> @@ -733,6 +733,11 @@ unsigned long max_sane_readahead(unsigne

Quite simple patch, why is it that much simpler than your earlier patchkits?
Or is that on top of them?

You seem to have a lot of magic numbers. They probably all need symbols and 
explanations.

Your white space also needs some work.

-Andi
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