linux-kernel - Re: [PATCH 2/9] readahead: snap readahead request to EOF

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Date:	Tue, 29 Nov 2011 15:29:58 +0100
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/9] readahead: snap readahead request to EOF

On Tue 29-11-11 21:09:02, Wu Fengguang wrote:
> If the file size is 20kb and readahead request is [0, 16kb),
> it's better to expand the readahead request to [0, 20kb), which will
> likely save one followup I/O for [16kb, 20kb).
> 
> If the readahead request already covers EOF, trimm it down to EOF.
> Also don't set the PG_readahead mark to avoid an unnecessary future
> invocation of the readahead code.
> 
> This special handling looks worthwhile because small to medium sized
> files are pretty common.
> 
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> ---
>  mm/readahead.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> --- linux-next.orig/mm/readahead.c	2011-11-29 11:28:56.000000000 +0800
> +++ linux-next/mm/readahead.c	2011-11-29 11:29:05.000000000 +0800
> @@ -251,8 +251,16 @@ unsigned long max_sane_readahead(unsigne
>  unsigned long ra_submit(struct file_ra_state *ra,
>  		       struct address_space *mapping, struct file *filp)
>  {
> +	pgoff_t eof = ((i_size_read(mapping->host)-1) >> PAGE_CACHE_SHIFT) + 1;
> +	pgoff_t start = ra->start;
>  	int actual;
>  
> +	/* snap to EOF */
> +	if (start + ra->size + ra->size / 2 > eof) {
> +		ra->size = eof - start;
> +		ra->async_size = 0;
> +	}
> +
>  	actual = __do_page_cache_readahead(mapping, filp,
>  					ra->start, ra->size, ra->async_size);
  Hmm, wouldn't it be cleaner to do this already in ondemand_readahead()?
All other updates of readahead window seem to be there. Also shouldn't we
take maximum readahead size into account? Reading 3/2 of max readahead
window seems like a relatively big deal for large files...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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