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    SubjectRe: [PATCH 5/8] readahead: add /debug/readahead/stats
    On Tue, 29 Nov 2011 11:23:23 +0800 Wu Fengguang <fengguang.wu@intel.com> wrote:

    > > > +{
    > > > +#ifdef CONFIG_READAHEAD_STATS
    > > > + if (readahead_stats_enable) {
    > > > + readahead_stats(mapping, offset, req_size, ra_flags,
    > > > + start, size, async_size, actual);
    > > > + readahead_stats(mapping, offset, req_size,
    > > > + RA_PATTERN_ALL << READAHEAD_PATTERN_SHIFT,
    > > > + start, size, async_size, actual);
    > > > + }
    > > > +#endif
    > > > +}
    > >
    > > The stub should be inlined, methinks. The overhead of evaluating and
    > > preparing eight arguments is significant. I don't think the compiler
    > > is yet smart enough to save us.
    >
    > The parameter list actually becomes even out of control when doing the
    > bit fields:
    >
    > + readahead_event(mapping, offset, req_size,
    > + ra->pattern, ra->for_mmap, ra->for_metadata,
    > + ra->start + ra->size >= eof,
    > + ra->start, ra->size, ra->async_size, actual);
    >
    > So I end up passing file_ra_state around. The added cost is, I'll have
    > to dynamically create a file_ra_state for the fadvise case, which
    > should be acceptable since it's a cold path.

    That will reduce the cost of something which would have zero cost by
    making this function a static inline when CONFIG_READAHEAD_STATS=n.



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