Re: Minutes from 10/1 LSE Call

[Andrew Morton]

Hanna Linder <hannal@xxxxxxxxxx> 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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