Messages in this thread | | | Date | Tue, 16 Feb 2010 03:21:27 +1100 | From | Nick Piggin <> | Subject | Re: [PATCH 2/3] mm: Implement writeback livelock avoidance using page tagging |
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On Mon, Feb 15, 2010 at 04:47:51PM +0100, Jan Kara wrote: > On Fri 12-02-10 11:39:55, Andrew Morton wrote: > > On Fri, 12 Feb 2010 00:06:23 +0100 > > Jan Kara <jack@suse.cz> wrote: > > > > > The idea is simple: Tag all pages that should be written back > > > with a special tag (TOWRITE) in the radix tree. This can be done > > > rather quickly and thus livelocks should not happen in practice. > > > Then we start doing the hard work of locking pages and sending > > > them to disk only for those pages that have TOWRITE tag set. > > > > Adding a second pass across all the pages sounds expensive? > Strictly speaking it's just through the radix tree and only through > branches with DIRTY_TAG set. But yes, there is some additional CPU cost. > I just thought that given the total cost of submitting a page it is > an acceptable increase and the simplification is worth it. > Would some numbers make you happier? Any suggestion for measurements? > Because I think that even for writes to tmpfs the change will be lost > in the noise...
Although hmm, if it is a very large file with *lots* of dirty pages then it might become a noticable proportion of the cost.
Dave Chinner would probably tell you he's seen files with many gigabytes dirty, and what is nr_to_write set to? 1024 is it? So you might be tagging hundreds or thousands of radix tree entries per page you write.
Also, I wonder what you think about leaving the tags dangling when the loop bails out early? I have a *slight* concern about this because previously we never have a tag set when radix_tree_delete is called. I actually had a bug in that code in earlier versions of rcu radix tree that only got found by the user test harness. And another slight concern that it is just a bit ugly to leave the tag. But I can accept that lower CPU overhead trumps ugliness :)
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