Messages in this thread | | | Date | Wed, 29 Jun 2011 13:55:34 -0400 | From | Christoph Hellwig <> | Subject | Re: [PATCH] writeback: Don't wait for completion in writeback_inodes_sb_nr |
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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.
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