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SubjectRe: [PATCH] writeback: Fix broken sync writeback
On Tue 16-02-10 15:34:01, Linus Torvalds wrote:
> On Wed, 17 Feb 2010, Jan Kara wrote:
> >
> > The IO size actually does matter for performance because if you switch
> > after 4 MB (current value of MAX_WRITEBACK_PAGES) to writing another inode,
>
> No.
>
> Dammit, read the code.
>
> That's my whole _point_. Look at the for-loop.
>
> We DO NOT SWITCH to another inode, because we just continue in the
> for-loop.
>
> This is why I think your patch is crap. You clearly haven't even read the
> code, the patch makes no sense, and there must be something else going on
> than what you _claim_ is going on.
I've read the code. Maybe I'm missing something but look:
writeback_inodes_wb(nr_to_write = 1024)
-> queue_io() - queues inodes from wb->b_dirty list to wb->b_io list
...
writeback_single_inode()
...writes 1024 pages.
if we haven't written everything in the inode (more than 1024 dirty
pages) we end up doing either requeue_io() or redirty_tail(). In the
first case the inode is put to b_more_io list, in the second case to
the tail of b_dirty list. In either case it will not receive further
writeout until we go through all other members of current b_io list.

So I claim we currently *do* switch to another inode after 4 MB. That
is a fact.

I *think* it is by design - mainly to avoid the situation where someone
continuously writes a huge file and kupdate or pdflush would never get to
writing other files with dirty data (at least that's impression I've built
over the years - heck, even 2.6.16 seems to have this redirty_tail logic
with a comment about the above livelock).

I do find this design broken as well as you likely do and think that the
livelock issue described in the above paragraph should be solved differently
(e.g. by http://lkml.org/lkml/2010/2/11/321) but that's not a quick fix.

The question is what to do now for 2.6.33 and 2.6.32-stable. Personally,
I think that changing the writeback logic so that it does not switch inodes
after 4 MB is too risky for these two kernels. So with the above
explanation would you accept some fix along the lines of original Jens'
fix?

Honza

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


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