On Tue, 07/14 13:31, Stefan Hajnoczi wrote: > On Fri, Jul 10, 2015 at 05:42:48AM +0200, Alexandre DERUMIER wrote: > > >>By the way, why did you choose 10 milliseconds? That is quite long. > > >> > > >>If this function is called once per 10 ms disk I/O operations then we > > >>lose 50% utilization. 1 ms or less would be reasonable. > > > > From my tests, 1ms is not enough, It still hanging in guest or qmp queries. > > 10ms give me optimal balance between bitmap scan speed and guest > > responsiveness. > > Then I don't fully understand the bug. > > Fam: can you explain why 1ms isn't enough?
In Alexandre's case, I suppose it's because the lseek is so slow that sleeping for 1ms would still let mirror coroutine to occupy, say, 90% of CPU time, so guest IO stutters. Perhaps we could move lseek to thread pool in the future. Anyway, 10ms wasn't a deliberate choice, because I didn't have one. I agree in other cases, 1ms or less should be enough. Fam