linux-kernel - Re: PATCH] fix potential latency issues in JBD's journal code

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Message-ID: <20080705223021.02735dc0@infradead.org>
Date:	Sat, 5 Jul 2008 22:30:21 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Theodore Tso <tytso@....EDU>
Cc:	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: PATCH] fix potential latency issues in JBD's journal code

On Sun, 6 Jul 2008 00:15:02 -0400
Theodore Tso <tytso@....EDU> wrote:

> On Fri, Jul 04, 2008 at 07:29:29PM -0700, Arjan van de Ven wrote:
> > @@ -165,7 +165,7 @@ static void journal_do_submit_data(struct
> > buffer_head **wbuf, int bufs) for (i = 0; i < bufs; i++) {
> >  		wbuf[i]->b_end_io = end_buffer_write_sync;
> >  		/* We use-up our safety reference in submit_bh() */
> > -		submit_bh(WRITE, wbuf[i]);
> > +		submit_bh(WRITE_SYNC, wbuf[i]);
> >  	}
> >  }
> 
> So I started looking at this patch more closely when trying to
> replicate it for ext4.  Don't you want to only use WRITE_SYNC() only
> for the very last time in the loop?  Otherwise you end up unplugging
> the queue after each bufferhead, which wouldn't be a good thing,
> right?

it's debatable. Because this submit-bh() will sometimes block... it
wouldn't get IO started for that case. I think this is where I'd like
Jens to say what he thinks the rules are; clearly the elevator needs to
treat all of these guys as sync, but maybe the plugging should be ..
flexible.


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