Re: [take36 10/10] kevent: Kevent based generic AIO.

Evgeniy Polyakov
2007-02-12T12:19:40Z

On Mon, Feb 12, 2007 at 02:08:10PM +0100, Andi Kleen (ak@suse.de) wrote:
> Evgeniy Polyakov <johnpol@2ka.mipt.ru> writes:
> >
> > aio_sendfile_path() is essentially aio_sendfile(), except that it takes
> > source filename as parameter, has a pointer to private header
> > and its size (which allows to send header and file's content in one syscall
> > instead of three (open, send, sendfile) and returns opened file descriptor.
>
> Are you sure this is a useful optimization? Do you have numbers vs open+aio_sendfile+close?
>
> Compared to the cost of sending a complete file three system calls should be quite in the noise.
> And Linux system calls are not that expensive (few hundred cycles normally)
>
> Adding such compound system calls would be a worrying precedent because
> I'm sure others would want them then for their favourite system call combo
> too. If they were really useful it might make more sense to have a batch()
> system call that works for arbitary calls, but I'm not convinced yet
> it's even needed. It would be certainly ugly.

Yes, that call ends up about 10MB/sec faster for 100 1mb file transfers
over 1gbit network (78 MB/s vs 66-72 MB/s over 1 Gb network, sendfile sending server
is one-way AMD Athlong 64 3500+), but indeed, it can be the case that async IO sending
was main speed factor.

I added header by request from Suparna Bhattacharya - my main position
is the same about syscall overhead, but I do not that care.

> -Andi

--
Evgeniy Polyakov