Messages in this thread | | | Date | Mon, 28 Sep 2009 10:25:24 -0400 | From | Chris Mason <> | Subject | Re: regression in page writeback |
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On Mon, Sep 28, 2009 at 11:07:00AM +1000, Dave Chinner wrote: > On Fri, Sep 25, 2009 at 02:45:03PM +0800, Wu Fengguang wrote: > > On Fri, Sep 25, 2009 at 01:04:13PM +0800, Dave Chinner wrote: > > > On Thu, Sep 24, 2009 at 08:38:20PM -0400, Chris Mason wrote: > > > > On Fri, Sep 25, 2009 at 10:11:17AM +1000, Dave Chinner wrote: > > > > > On Thu, Sep 24, 2009 at 11:15:08AM +0800, Wu Fengguang wrote: > > > > > > On Wed, Sep 23, 2009 at 10:00:58PM +0800, Chris Mason wrote: > > > > > > > The only place that actually honors the congestion flag is pdflush. > > > > > > > It's trivial to get pdflush backed up and make it sit down without > > > > > > > making any progress because once the queue congests, pdflush goes away. > > > > > > > > > > > > Right. I guess that's more or less intentional - to give lowest priority > > > > > > to periodic/background writeback. > > > > > > > > > > IMO, this is the wrong design. Background writeback should > > > > > have higher CPU/scheduler priority than normal tasks. If there is > > > > > sufficient dirty pages in the system for background writeback to > > > > > be active, it should be running *now* to start as much IO as it can > > > > > without being held up by other, lower priority tasks. > > > > > > > > I'd say that an fsync from mutt or vi should be done at a higher prio > > > > than a background streaming writer. > > > > > > I don't think you caught everything I said - synchronous IO is > > > un-throttled. > > > > O_SYNC writes may be un-throttled in theory, however it seems to be > > throttled in practice: > > > > generic_file_aio_write > > __generic_file_aio_write > > generic_file_buffered_write > > generic_perform_write > > balance_dirty_pages_ratelimited > > generic_write_sync > > > > Do you mean some other code path? > > In the context of the setup I was talking about, I meant is that sync > IO _should_ be unthrottled because it is self-throttling by it's > very nature. The current code makes no differentiation between the > two.
This isn't entirely true anymore. WB_SYNC_ALL is turned into a sync bio, which is sent down with higher priority. There may be a few spots that still need to be changed for it, but it is much better than it was.
re: pageout() being the worst way to do IO, definitely agree.
-chris
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