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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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