linux-kernel - Re: [PATCH 12/13] writeback: try more writeback as long as something was written

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Date:	Fri, 6 Aug 2010 00:50:13 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	Mel Gorman <mel@....ul.ie>,
	Chris Mason <chris.mason@...cle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 12/13] writeback: try more writeback as long as
 something was written

On Fri 06-08-10 06:39:29, Wu Fengguang wrote:
> On Fri, Aug 06, 2010 at 01:00:16AM +0800, Jan Kara wrote:
> > I'm just afraid that in some
> > pathological cases this could result in bad writeback pattern - like if
> > there is some process which manages to dirty just a few pages while we are
> > doing writeout, this looping could result in writing just a few pages in
> > each round which is bad for fragmentation etc.
> 
> Such inodes will be redirty_tail()ed here:
> 
>                 if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
>                         /*
>                          * We didn't write back all the pages.  nfs_writepages()
>                          * sometimes bales out without doing anything.
>                          */
>                         inode->i_state |= I_DIRTY_PAGES;
>                         if (wbc->nr_to_write <= 0) {
>                                 /*
>                                  * slice used up: queue for next turn
>                                  */
>                                 requeue_io(inode);
>                         } else {
>                                 /*
>                                  * Writeback blocked by something other than
>                                  * congestion. Delay the inode for some time to
>                                  * avoid spinning on the CPU (100% iowait)
>                                  * retrying writeback of the dirty page/inode
>                                  * that cannot be performed immediately.
>                                  */
>                                 redirty_tail(inode);
>                         }
  Yes. And then, when there are no inodes in b_more_io, they get queued
again for writeback. So for non-background WB_SYNC_NONE writeback we can
just write a few pages over and over again... Oh, ok we won't because of
my start_time fix I suppose. Maybe a comment about this by the nr_to_write
< MAX_WRITEBACK_PAGES check would be good.

> >   Actually, this comment probably also applies to your patch where you
> > change the queueing logic in writeback_single_inode(), doesn't it?
> 
> Can you elaborate?
  Sorry, my comment only applies to this particular patch. In your change
to writeback_single_inode() you requeue_io() only if nr_to_write <= 0.

								Honza
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