Date: Wed, 29 Jun 2011 21:15:18 +0200
From: Jan Kara <jack@suse.cz>
To: Christoph Hellwig <hch@infradead.org>
Cc: Jan Kara <jack@suse.cz>, Curt Wohlgemuth <curtw@google.com>,
        Al Viro <viro@zeniv.linux.org.uk>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, fengguang.wu@intel.com
Subject: Re: [PATCH] writeback: Don't wait for completion in
 writeback_inodes_sb_nr
Message-ID: <20110629191518.GA23196@quack.suse.cz>
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On Wed 29-06-11 13:55:34, Christoph Hellwig wrote:
> On Wed, Jun 29, 2011 at 06:57:14PM +0200, Jan Kara wrote:
> > > For sys_sync I'm pretty sure we could simply remove the
> > > writeback_inodes_sb call and get just as good if not better performance,
> >   Actually, it won't with current code. Because WB_SYNC_ALL writeback
> > currently has the peculiarity that it looks like:
> >   for all inodes {
> >     write all inode data
> >     wait for inode data
> >   }
> > while to achieve good performance we actually need something like
> >   for all inodes
> >     write all inode data
> >   for all inodes
> >     wait for inode data
> > It makes a difference in an order of magnitude when there are lots of
> > smallish files - SLES had a bug like this so I know from user reports ;)
> 
> I don't think that's true.  The WB_SYNC_ALL writeback is done using
> sync_inodes_sb, which operates as:
> 
>   for all dirty inodes in bdi:
>      if inode belongs to sb
>         write all inode data
> 
>   for all inodes in sb:
>      wait for inode data
> 
> we still do that in a big for each sb loop, though.
  True but writeback_single_inode() has in it:
        if (wbc->sync_mode == WB_SYNC_ALL) {
                int err = filemap_fdatawait(mapping);
                if (ret == 0)
                        ret = err;
        }
  So we end up waiting much earlier. Probably we should remove this wait
but that will need some audit I guess.

> >   You mean that sync(1) would actually write the data itself? It would
> > certainly make some things simpler but it has its problems as well - for
> > example sync racing with flusher thread writing back inodes can create
> > rather bad IO pattern...
> 
> Only the second pass.  The idea is that we first try to use the flusher
> threads for good I/O patterns, but if we can't get that to work only
> block the caller and not everyone.  But that's just an idea so far,
> it would need serious benchmark.  And despite what I claimed before
> we actually do the wait in the caller context already anyway, which
> already gives you the easy part of the above effect.
  Modulo the writeback_single_inode() wait. But if that is dealt with I
agree.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR
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