linux-kernel - Re: splice/vmsplice performance test results

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Message-Id: <20061116.162748.102576265.davem@davemloft.net>
Date:	Thu, 16 Nov 2006 16:27:48 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	jens.axboe@...cle.com
Cc:	jaschut@...dia.gov, linux-kernel@...r.kernel.org
Subject: Re: splice/vmsplice performance test results

From: Jens Axboe <jens.axboe@...cle.com>
Date: Thu, 16 Nov 2006 22:21:07 +0100

> On Thu, Nov 16 2006, David Miller wrote:
> > From: "Jim Schutt" <jaschut@...dia.gov>
> > Date: Thu, 16 Nov 2006 11:08:59 -0700
> > 
> > > Or is read+write really the fastest way to get data off a
> > > socket and into a file?
> > 
> > There is still no explicit TCP support for splice/vmsplice so things
> > get copied around and most of the other advantaves of splice/vmplice
> > aren't obtained either.  So perhaps that explains your numbers.
> 
> There should not be any copying for tcp send, at least no more than what
> sendfile() did/does. Hmm?

That's true on send, correct.

> > Jens Axboe tries to get things working, and others have looked into it
> > too, but adding TCP support is hard and for several reasons folks like
> > Alexey Kuznetsov and Evgeniy Polyakov believe that sys_receivefile()
> > is an interface much better suited for TCP receive.
> > 
> > splice/vmsplice has a lot of state connected to a transaction, and
> > perhaps that is part of why Evgeniy and Alexey have trouble wrapping
> > their brains around an efficient implementation.
> 
> I hope to try and see if I can help get some of that done, however I
> need all the help I can get on the networking side. Not sure I
> understand why it has to be so difficult, if we need to define a wrapper
> container instead of passing down a pipe that is completely fine with
> me. The networking code basically just needs to hang on to the
> pipe_buffer and release it on ack for send, receive is somewhat more
> involved (and I don't know enough about networking to voice any
> half-intelligent opinion on that!).
> 
> I would just consider it a damn shame if we cannot complete the splice
> family and need to punt to something else for net receive.

I'm sure that the folks on netdev@...r.kernel.org are more than
willing to give you a hand in this area :-)
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