linux-kernel - Re: Major network performance regression in 3.7

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Date:	Sun, 6 Jan 2013 16:55:20 -0500
From:	"John Stoffel" <john@...ffel.org>
To:	Willy Tarreau <w@....eu>
Cc:	John Stoffel <john@...ffel.org>,
	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Major network performance regression in 3.7

>>>>> "Willy" == Willy Tarreau <w@....eu> writes:

Willy> On Sun, Jan 06, 2013 at 04:49:35PM -0500, John Stoffel wrote:
>> >>>>> "Willy" == Willy Tarreau <w@....eu> writes:
>> 
Willy> On Sun, Jan 06, 2013 at 11:00:15AM -0800, Eric Dumazet wrote:
>> >> On Sun, 2013-01-06 at 10:51 -0800, Eric Dumazet wrote:
>> >> > > 
>> >> > > (sd->len is usually 4096, which is expected, but sd->total_len value is
>> >> > > huge in your case, so we always set the flag in fs/splice.c)
>> >> > 
>> >> > I am testing :
>> >> > 
>> >> >        if (sd->len < sd->total_len && pipe->nrbufs > 1)
>> >> >                 more |= MSG_SENDPAGE_NOTLAST;
>> >> > 
>> >> 
>> >> Yes, this should fix the problem :
>> >> 
>> >> If there is no following buffer in the pipe, we should not set NOTLAST.
>> >> 
>> >> diff --git a/fs/splice.c b/fs/splice.c
>> >> index 8890604..6909d89 100644
>> >> --- a/fs/splice.c
>> >> +++ b/fs/splice.c
>> >> @@ -696,8 +696,10 @@ static int pipe_to_sendpage(struct pipe_inode_info *pipe,
>> >> return -EINVAL;
>> >> 
>> >> more = (sd->flags & SPLICE_F_MORE) ? MSG_MORE : 0;
>> >> -	if (sd->len < sd->total_len)
>> >> +
>> >> +	if (sd->len < sd->total_len && pipe->nrbufs > 1)
>> >> more |= MSG_SENDPAGE_NOTLAST;
>> >> +
>> >> return file->f_op->sendpage(file, buf->page, buf->offset,
sd-> len, &pos, more);
>> >> }
>> 
Willy> OK it works like a charm here now ! I can't break it anymore, so it
Willy> looks like you finally got it !
>> 
>> It's still broken, there's no comments in the code to explain all this
>> magic to mere mortals!  *grin*

Willy> I would generally agree, but when Eric fixes such a thing, he
Willy> generally goes with lengthy details in the commit message.

I'm sure he will too, I just wanted to nudge him because while I sorta
followed this discussion, I see lots of pain down the road if the code
wasn't updated with some nice big fat comments.

Great job finding this code and testing, testing, testing.

John

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