Re: Minutes from 10/1 LSE Call

[Jeff Garzik <jgarzik@pobox.com>]

Larry McVoy wrote:
> On Wed, Oct 01, 2003 at 04:29:16PM -0700, Andrew Morton wrote:
> 
>>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.
> 
> 
> I've seen this too, not that Andrew needs me to back him up, but in many 
> cases even 4k is big enough.  Linux has a very thin system call layer so
> it is OK, good even, to use reasonable buffer sizes.


Slight tangent, FWIW...   Back when I was working on my "race-free 
userland" project, I noticed that the fastest cp(1) implementation was 
GNU's:  read/write from a single, statically allocated, page-aligned 4K 
buffer.  I experimented with various buffer sizes, mmap-based copies, 
and even with sendfile(2) where both arguments were files. 
read(2)/write(2) of a single 4K buffer was always the fastest.

	Jeff