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SubjectRe: [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@intel.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@suse.cz>
SUSE Labs, CR


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