Messages in this thread | | | Date | Wed, 23 Sep 2009 14:41:20 +0800 | From | Shaohua Li <> | Subject | Re: regression in page writeback |
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On Tue, Sep 22, 2009 at 11:52:59PM +0800, Chris Mason wrote: > On Tue, Sep 22, 2009 at 10:32:14AM +0200, Peter Zijlstra wrote: > > On Tue, 2009-09-22 at 16:24 +0800, Wu Fengguang wrote: > > > On Tue, Sep 22, 2009 at 04:09:25PM +0800, Peter Zijlstra wrote: > > > > On Tue, 2009-09-22 at 16:05 +0800, Wu Fengguang wrote: > > > > > > > > > > I'm not sure how this patch stopped the "overshooting" behavior. > > > > > Maybe it managed to not start the background pdflush, or the started > > > > > pdflush thread exited because it found writeback is in progress by > > > > > someone else? > > > > > > > > > > - if (bdi_nr_reclaimable) { > > > > > + if (bdi_nr_reclaimable > bdi_thresh) { > > > > > > > > The idea is that we shouldn't move more pages from dirty -> writeback > > > > when there's not actually that much dirty left. > > > > > > IMHO this makes little sense given that pdflush will move all dirty > > > pages anyway. pdflush should already be started to do background > > > writeback before the process is throttled, and it is designed to sync > > > all current dirty pages as quick as possible and as much as possible. > > > > Not so, pdflush (or now the bdi writer thread thingies) should not > > deplete all dirty pages but should stop writing once they are below the > > background limit. > > > > > > Now, I'm not sure about the > bdi_thresh part, I've suggested to maybe > > > > use bdi_thresh/2 a few times, but it generally didn't seem to make much > > > > of a difference. > > > > > > One possible difference is, the process may end up waiting longer time > > > in order to sync write_chunk pages and quit the throttle. This could > > > hurt the responsiveness of the throttled process. > > > > Well, that's all because this congestion_wait stuff is borken.. > > > > I'd suggest retesting with a new baseline against the code in Linus' git > today. Overall I think the change to make balance_dirty_pages() sleep > instead of kick more IO out is a very good one. It helps in most > workloads here. I tested today's git tree v2.6.31-7068-g43c1266, looks the regression disappears. the disk io is almost stable to about 480m/s with/without the patch.
Thanks, Shaohua
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