Zero-copy patch

[Jacky Lam]

> -----Original Message-----
> From: Jeremy Allison [mailto:jra at samba.org]
> Sent: Thursday, February 10, 2011 9:43 AM
> To: Jacky Lam
> Cc: Jeremy Allison; Volker.Lendecke at SerNet.DE; samba-
> technical at lists.samba.org
> Subject: Re: Zero-copy patch
>
> On Tue, Feb 08, 2011 at 01:53:14AM -0500, Jacky Lam wrote:
> > Jeremy,
> >
> >         After tracing the kernel code, I come up with a disappointing
> conclusion.
> >
> >         The heavy memory copy are coming from two places in kernel:
> >
> >         1. net/core/skbuff.c: linear_to_page()
> >         2. fs/splice.c: pipe_to_file()
> >
> >         The first one is actually introduced in 2.6.28.6/2.6.29 to
> fix a data corruption bug when splice from socket to socket. I quote
> the kernel change log at the end of mail. This one seems don't bother
> Samba, so I roll back the fix and get a 10% improvement.
> >
> >         The second one do memcopy when
> >                 *       Destination page already exists in the
> address space and there
> >                 *       are users of it. For that case we have no
> other option that
> >                 *       copying the data. Tough luck.
> >         But I am curious who the users are (mistake?). So, I just
> comment out the memory copy and expect data corruption. I want to check
> what can I achieve if all these things are probably fixed.
> >
> >         Finally, I get the throughput more or less the same as using
> read()/write(). It doesn't worthwhile to bother all those trouble at
> all. Maybe the data flow of current implementation of socket splice are
> inefficient enough indeed.
> >
> >         If there is not any other idea, I think I would rather spend
> time on how to make the patches you send me to work.
>
> Another OEM (who shall remain nameless) mentioned
> that changing the kernel scheduler to the deadline
> scheduler helps performance (you may already have
> done this of course).
>
> Let me know if you haven't and if it helps.
>
> Jeremy.

I have tried before and just now again. No noticeable changes.

BTW, I applied the patch to kernel of my desktop. The patch is applied cleanly but obviously missing something. It add splice_direct_from_socket() for splice(), but have nothing change for sendfile(). Anyway, I do a quick fix for that and finally got sendfile() do what we want. The result is the throughput is about 5% faster than read/write implementation. But it is far lower than my expectation. Is this reported result of the patch?

Jacky


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