linux-kernel - Re: splice() on two pipes

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Message-ID: <20090430045645.GF570@1wt.eu>
Date:	Thu, 30 Apr 2009 06:56:47 +0200
From:	Willy Tarreau <w@....eu>
To:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org
Subject: Re: splice() on two pipes

On Wed, Apr 29, 2009 at 09:42:55PM +0200, Max Kellermann wrote:
> On 2009/04/29 17:23, Andi Kleen <andi@...stfloor.org> wrote:
> > I don't think splice is about handling all possible cases,
> > but just cases where the kernel can do better than user space.
> > I don't think that's the case here.
> 
> If splice() is about passing pointers of a pipe buffer, what's more
> trivial (and natural) than passing that pointer between two pipes?
> 
> > > when I read about the splice() system call, I thought it was obvious
> > > that it could copy data between two pipes. 
> > 
> > It would be more efficient if you used fd passing to pass the fd
> > around to the other process and let it read directly.
> 
> That's not so easy in my case.  The header output of the one process
> has to be parsed before the rest of it (or part of the rest) is going
> to be forwarded to the second one.  My master process would lose
> control over the transfer.  splice() looks like the perfect solution.

indeed, that could make sense. From what I have seen in the splicing
code, I think tht implementing pipe to pipe should not be *that* hard,
starting from existing code (eg: net to pipe). Maybe you could try to
implement it since you have the code which makes use of it ? I think
it is the kind of feature which can only improve step by step based
on application needs.

BTW, I like your test program. Simple and easy. I have completed it to
test tcp and udp, you can find it attached.

Regards,
Willy


View attachment "test_splice.c" of type "text/plain" (3241 bytes)