Linux-kernel mailing list archive 1998-25,: Re: Thread implementations...

# Re: Thread implementations...

Stephen C. Tweedie (sct@dcs.ed.ac.uk)
Fri, 26 Jun 1998 10:30:21 +0100

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Hi,

On Fri, 26 Jun 1998 07:19:47 +0200 (MET DST), MOLNAR Ingo
<mingo@valerie.inf.elte.hu> said:

>> I can't see any advantage of sendfile() over push/pull other than a
>> reduction in the number of system calls required to get the job done.

> no there is a much more important difference in favor of sendfile(), it's
> the number of 'cursors' active within the system, and the timing of the
> movement of these cursors and the coupling between them.

> Timing: eg., alone from the memory consumption point of view it's
> perfectly possible to sendfile() a 500M file. With pull()+push(), we first
> read it into the page cache, then write it out, we go through the page
> cache twice.

If you are using the page cache, then true. There's no reason why
pull and push _have_ to use the page cache, but it's probably simpler
to implement that way. Likewise, there's no reason why sendfile()
can't use it; again, however, the easiest implementation will probably
do so.

> Coupling: much more important is the cache-utilization argument, if you
> sufficiently slice up the work done within sendfile(), you will have the
> 'freshly read' data still in some of the caches.

CPU cache, is this? Either sendfile() or pull/push will do the IO
without any copies at all (assuming any remotely current motherboard
with either scsi or dma-enabled ide) when reading from disk. The
write (if it's to network, rather than to disk) is going to have to
prime the cache either way; there's no difference between sendfile and
pull/push from that respect.

--Stephen

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