Messages in this thread | | | Date | Wed, 1 Oct 2003 16:29:16 -0700 | From | Andrew Morton <> | Subject | Re: Minutes from 10/1 LSE Call |
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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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