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    SubjectRe: [Lse-tech] Re: [RFC] dcache scalability patch (2.4.17)

    On Fri, 12 Jul 2002, Paul Menage wrote:

    > >Note: measurements on 2.4 do not make sense; reduction of cacheline
    > >bouncing between 2.4 and 2.5 will change the results anyway and
    > >if any of these patches are going to be applied to 2.4, reduction of
    > >cacheline bouncing on ->d_count is going to go in before that one.
    > I think there are some other possibilities for cache-bounce removal in
    > struct dentry. The most obvious one is d_vfs_flags - not only does it
    > get written to on every d_lookup (to set DCACHE_REFERENCED) but it also
    > shares a cache line (on Pentium IV) with d_op, d_iname and part of
    > d_name (along with d_sb and d_fsdata, but these don't seem to be so
    > crucial).
    > Some quick stats gathering suggested that DCACHE_REFERENCED is already
    > set 95%-98% of the time, so this cache bounce is not even doing anything
    > useful. I submitted this patch a while ago making the DCACHE_REFERENCED
    > bit setting be conditional on it not being already set, which didn't
    > generate any interest. One problem with this patch would be the
    > additional branch prediction misses (on some architectures?) that would
    > work against the benefits of not dirtying a cache line.

    Frankly, I'd rather moved setting DCACHE_REFERENCED to dput(). We don't
    care for that bit for dentries with positive ->d_count.

    So I'd just do

    vi fs/dcache.c -c '/|= DCACHE_R/d|/nr_un/pu|<|x'

    and be done with that. Linus?

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