On Tue, 29 Jul 2014 21:48:34 -0400 Milosz Tanski <milosz@xxxxxxxxx> wrote: > I would vote on the lower end of the spectrum by default (closer to > 100ms) since I imagine anybody deploying this in production > environment would likely be using SSD drives for the caching. And in > my tests on spinning disks there was little to no benefit outside of > reducing network traffic. Maybe I'm confused...... I thought the whole point of this patch was to avoid deadlocks. Now you seem to be talking about a performance benefit. What did I miss? NeilBrown > > - Milosz > > On Tue, Jul 29, 2014 at 5:17 PM, NeilBrown <neilb@xxxxxxx> wrote: > > On Tue, 29 Jul 2014 17:12:34 +0100 David Howells <dhowells@xxxxxxxxxx> wrote: > > > >> Milosz Tanski <milosz@xxxxxxxxx> wrote: > >> > >> > That's the same thing exact fix I started testing on Saturday. I found that > >> > there already is a wait_event_timeout (even without your recent changes). The > >> > thing I'm not quite sure is what timeout it should use? > >> > >> That's probably something to make an external tuning knob for. > >> > >> David > > > > Ugg. External tuning knobs should be avoided wherever possible, and always > > come with detailed instructions on how to tune them </rant> > > > > In this case I think it very nearly doesn't matter *at all* what value is > > used. > > > > If you set it a bit too high, then on the very very rare occasion that it > > would currently deadlock, you get a longer-than-necessary wait. So just make > > sure that is short enough that by the time the sysadmin notices and starts > > looking for the problem, it will be gone. > > > > And if you set it a bit too low, then it will loop around to find another > > page to deal with before that one is finished being written out, and so maybe > > do a little bit more work than is needed (though it'll be needed eventually). > > > > So the perfect number is somewhere between the typical response time for > > storage, and the typical response time for the sys-admin. Anywhere between > > 100ms and 10sec would do. 1 second is the geo-mean. > > > > (sorry I didn't reply earlier - I missed you email somehow). > > > > NeilBrown > > >
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