linux-kernel - Re: [PATCH] Add block device speciffic splice write method

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Message-ID: <20081023071559.GV22217@kernel.dk>
Date:	Thu, 23 Oct 2008 09:16:00 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dmitri Monakhov <dmonakhov@...nvz.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] Add block device speciffic splice write method

On Thu, Oct 23 2008, Andrew Morton wrote:
> On Thu, 23 Oct 2008 08:51:13 +0200 Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> > On Wed, Oct 22 2008, Andrew Morton wrote:
> > > On Thu, 23 Oct 2008 08:29:23 +0200 Jens Axboe <jens.axboe@...cle.com> wrote:
> > > 
> > > > > But really it'd be better if the throttling happened down in
> > > > > pipe_to_file(), on a per-page basis.  As it stands we can dirty an
> > > > > arbitrary number of pagecache pages without throttling.  I think?
> > > > 
> > > > That's pretty exactly why it isn't done in the actor, to avoid doing it
> > > > per-page. As it's going to be PIPE_BUFFERS (16) pages max, I think this
> > > > is better.
> > > > 
> > > > Back in the splice early days, the balance_dirty_pages() actually showed
> > > > up in profiles when it was done on a per-page basis. So I'm reluctant to
> > > > change it :-)
> > > 
> > > That's why (the misnamed) balance_dirty_pages_ratelimited() exists?
> > 
> > I think that is what was used, but the details are a little hazy at this
> > point. So I can't say for sure.
> 
> All that function does is to bump a per-cpu variable and
> once-per-thousand or so it does the balance.  If it was causing
> problems in the splice application we want to know, because write()
> uses it!

Once per 8 or 32. If we haven't exceeded the dirty limit, calling it in
the actor or at the end should not make a difference for splice, since
we should be going into balance_dirty_pages() at most once.

Perhaps it was different some years ago, or perhaps the micro benchmarks
were screwed. Or perhaps my memory is shot, can't say for sure :)

> >  In this case it's moot anyway, since we can kill it.
> 
> Nope, we can only remove it if the fd is O_SYNC||is_sync().

Right, I forgot this is still the buffered path.

-- 
Jens Axboe

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