Re: [PATCH] writeback: Don't wait for completion in writeback_inodes_sb_nr

[Christoph Hellwig <hch@infradead.org>]

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.

>   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.