linux-kernel - Re: using splice/vmsplice to improve file receive performance

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Date:	Fri, 22 Dec 2006 12:39:17 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	saeed bishara <saeed.bishara@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: using splice/vmsplice to improve file receive performance

On Fri, Dec 22 2006, saeed bishara wrote:
> On 12/22/06, Jens Axboe <jens.axboe@...cle.com> wrote:
> >On Thu, Dec 21 2006, saeed bishara wrote:
> >> Hi,
> >> I'm trying to use the splice/vmsplice system calls to improve the
> >> samba server write throughput, but before touching the smbd, I started
> >> to improve the ttcp tool since it simple and has the same flow. I'm
> >> expecting to avoid the "copy_from_user" path when using those
> >> syscalls.
> >> so far, I couldn't make any improvement, actually the throughput get
> >> worst. the new receive flow looks like this (code also attached):
> >> 1. read tcp packet (64 pages) to page aligned buffer.
> >> 2. vmsplice the buffer to pipe with SPLICE_F_MOVE.
> >> 3. splice the pipe to the file, also with SPLICE_F_MOVE.
> >>
> >> the strace shows that the splice takes a lot of time. also when
> >> profiling the kernel, I found that the memcpy() called to often !!
> >
> >(didn't see this until now, axboe@...e.de doesn't work anymore)
> >
> >I'm assuming that you mean you vmsplice with SPLICE_F_GIFT, to hand
> >ownership of the pages to the kernel (in which case SPLICE_F_MOVE will
> >work, otherwise you get a copy)? If not, that'll surely cost you a data
> >copy
>   I'll try the vmplice with SPLICE_F_GIFT and splice with MOVE. btw,
> I noticed that the  splice system call takes the bulk of the time,
> does it mean anything?

Hard to say without seeing some numbers :-)

> >This sounds remarkably like a recent thread on lkml, you may want to
> >read up on that. Basically using splice for network receive is a bit of
> >a work-around now, since you do need the one copy and then vmsplice that
> >into a pipe. To realize the full potential of splice, we first need
> >socket receive support so you can skip that step (splice from socket to
> >pipe, splice pipe to file).
> Ashwini Kulkarni posted patches that implements that, see
> http://lkml.org/lkml/2006/9/20/272 .  is that right?
> >
> >There was no test code attached, btw.
> sorry, here it is.
> can you please add sample application to your test tools (splice,fio
> ,,) that demonstrates my flow; socket to file using read & vmsplice?

I didn't add such an example, since I had hoped that we would have
splice from socket support sooner rather than later. But I can do so, of
course.

I'll try your test. One thing that sticks out initially is that you
should be using full pages, the splice pipe will not merge page
segments. So don't use a buflen less than the page size.

-- 
Jens Axboe

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