Messages in this thread | | | Date | Wed, 17 Feb 2010 02:33:37 +0100 | From | Jan Kara <> | Subject | Re: [PATCH] writeback: Fix broken sync writeback |
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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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