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

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Date:	Thu, 16 Nov 2006 22:21:07 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	David Miller <davem@...emloft.net>
Cc:	jaschut@...dia.gov, linux-kernel@...r.kernel.org
Subject: Re: splice/vmsplice performance test results

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?

> 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.

-- 
Jens Axboe

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