Date: Sat, 17 Mar 2001 19:41:27 +0100 From: Maxime Henrion <mux@qualys.com> To: arch@FreeBSD.org Cc: Dag-Erling Smorgrav <des@ofug.org> Subject: Re: Proposal for a new syscall Message-ID: <20010317194127.D420@nebula.cybercable.fr> In-Reply-To: <xzpzoekb5zt.fsf@flood.ping.uio.no>; from des@ofug.org on Sat, Mar 17, 2001 at 07:33:10PM %2B0100 References: <20010317164411.A420@nebula.cybercable.fr> <xzpzoekcs3r.fsf@flood.ping.uio.no> <20010317173444.B420@nebula.cybercable.fr> <xzp4rwsco0r.fsf@flood.ping.uio.no> <20010317183137.C420@nebula.cybercable.fr> <xzpzoekb5zt.fsf@flood.ping.uio.no>
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Dag-Erling Smorgrav wrote:
> Maxime Henrion <mux@qualys.com> writes:
> > > > > > such a syscall in the kernel would allow to implement "zero-copy"
> > > > > > wherever it is feasible.
> > > > > No. It would save you two copies and a bunch of syscalls, but it
> > > > > wouldn't be real zero-copy, just "n-2 copy" instead of "n copy".
> > > > And if n == 2 ?
> > > It's never the case. I think the best you can do in userland is n = 3,
> > I'm talking about a syscall.
> Yes. I already told you that your proposed syscall would at best
> reduce the number of copies by two. Now I'm telling you that the
> minimum number of copies, without your proposed syscall, can't be less
> than 3. You do the math.
Ok.
> > Why couldn't it be zero-copy if sendfile() already does this ?
> Sendfile(2) doesn't do zero-copy, it does 2-copy (in the best of
> cases).
Thanks, I stand corrected. Perhaps that should be mentioned in the man
page ? Currently, it says this :
IMPLEMENTATION NOTES
The FreeBSD implementation of sendfile() is "zero-copy", meaning that it
has been optimized so that copying of the file data is avoided.
Maxime
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