Re: X much slower in 2.0.24 than in 1.2.13

Ray Van Tassle-CRV004 (Ray_Van_Tassle-CRV004@email.mot.com)
Wed, 6 Nov 1996 8:39:54 -0600

> > We hardly ever allocate a struct sock
> Since the last patch I see to sock.h just adds one field, but adds it in
> the middle rather than at the tail, that might be shoving something (or
> many somethings) off the end of a cache line... Has anyone tried
> optimizing the ordering for cache friendliness, or is it the way it is
> for some other reason?
>
> Keith

I played around with struct buffer_head (in fs.h), running heavy I/O
benchmark programs, and changing around the order of the members. Somewhere
around 1.2.86, another entry got added neat the top, completely destroying
sct's careful ordering of frequently-accessed entries.
My results: (TA-DA!!!) *None* *NADA*, absolutely no change in the overall
elapsed time, nor the system time, no matter how I shifted things around.
Not even when I tried to un-optimize things, and cross cache-line boundaries.

This was on a i486DX4/100, with both SCSI and IDE disks.

Ordering for cache access _may_ prove worthwhile, but my testing seems to
indicate that it probably isn't.

-30- Ray