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SubjectRe: Minutes from 10/1 LSE Call
Hanna Linder <hannal@us.ibm.com> wrote:
>
> In mainline, once block size is over 32k our throuput actually drops off.
> It levels off around 128k but at a greater cpu utilization.
> Dont really understand why that is.

Probably thrashing the CPU's L1 cache.

> In mm tree, maintains throughput for all block size but the cpu utilzation
> keeps going up to do the same throughput. Readprofile shows the extra time
> is being spent in copy_to_user (in mm tree). Backing out readahead patch
> reduces cpu by 10% for all block sizes but still shows the spike. So that
> isnt the main problem.

If you have a loop like:

char *buf;

for (lots) {
read(fd, buf, size);
}

the optimum value of `size' is small: as little as 8k. Once `size' gets
close to half the size of the L1 cache you end up pushing the memory at
`buf' out of CPU cache all the time.

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