Messages in this thread | | | Subject | Re: [RFC][PATCH] mm: stop balance_dirty_pages doing too much work | From | Richard Kennedy <> | Date | Fri, 07 Aug 2009 17:09:21 +0100 |
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On Fri, 2009-08-07 at 11:22 -0400, Chris Mason wrote: > On Fri, Aug 07, 2009 at 03:36:01PM +0100, Richard Kennedy wrote: > > On Fri, 2009-08-07 at 14:20 +0200, Peter Zijlstra wrote: > > > On Wed, 2009-06-24 at 11:38 +0100, Richard Kennedy wrote: > > ... > > > OK, so Chris ran into this bit yesterday, complaining that he'd only get > > > very few write requests and couldn't saturate his IO channel. > > > > > > Now, since writing out everything once there's something to do sucks for > > > Richard, but only writing out stuff when we're over the limit sucks for > > > Chris (since we can only be over the limit a little), the best thing > > > would be to only write out when we're over the background limit. Since > > > that is the low watermark we use for throttling it makes sense that we > > > try to write out when above that. > > > > > > However, since there's a lack of bdi_background_thresh, and I don't > > > think introducing one just for this is really justified. How about the > > > below? > > > > > > Chris how did this work for you? Richard, does this make things suck for > > > you again? > > > > > > --- > > > mm/page-writeback.c | 2 +- > > > 1 files changed, 1 insertions(+), 1 deletions(-) > > > > > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > > > index 81627eb..92f42d6 100644 > > > --- a/mm/page-writeback.c > > > +++ b/mm/page-writeback.c > > > @@ -545,7 +545,7 @@ static void balance_dirty_pages(struct address_space *mapping) > > > * threshold otherwise wait until the disk writes catch > > > * up. > > > */ > > > - if (bdi_nr_reclaimable > bdi_thresh) { > > > + if (bdi_nr_reclaimable > bdi_thresh/2) { > > My patch had two extra spaces ;) > > > > writeback_inodes(&wbc); > > > pages_written += write_chunk - wbc.nr_to_write; > > > get_dirty_limits(&background_thresh, &dirty_thresh, > > > > > > > > I'll run some tests and let you know :) > > > > But what if someone has changed the vm settings? > > Maybe something like > > (bdi_thresh * dirty_background_ratio / dirty_ratio) > > might be better ? > > > > Chris, what sort of workload are you having problems with? > > So, buffered writeback in general has a bunch of interesting features > right now, and to be honest I'm having a hard time untangling all of it. > It doesn't help that each of our filesystems is reacting differently. > > Btrfs and XFS both use helper threads to process IO completion. This > means that we tend to collect more writeback pages than the other > filesystems do. > > The end result of this is that O_DIRECT is dramatically faster than > buffered on every streaming write workload I've tried. I'm still trying > to sort out exactly where buffered IO is going wrong. > > -chris > Yes, it's all pretty complex. With a large number of pages in writeback do you think that the total dirty pages goes over the threshold ? I do wonder how long we get stuck in congestion_wait, it may be interesting to see if reducing its timeout has any effect.
I've been experimenting with rewriting balance_dirty_pages to reduce the number of calls to global_page_state. Those counters share a cacheline and are fairly hot. So far it's looking good & improving simple write performance particularly when the write size is just over the dirty threshold.
regard Richard
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