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@xxxxxxx>
SUSE Labs, CR
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