On Tue 03-05-16 08:23:27, Jens Axboe wrote: > On 05/03/2016 03:34 AM, Jan Kara wrote: > >On Thu 28-04-16 12:53:50, Jens Axboe wrote: > >>>2) As far as I can see in patch 8/8, you have plugged the throttling above > >>> the IO scheduler. When there are e.g. multiple cgroups with different IO > >>> limits operating, this throttling can lead to strange results (like a > >>> cgroup with low limit using up all available background "slots" and thus > >>> effectively stopping background writeback for other cgroups)? So won't > >>> it make more sense to plug this below the IO scheduler? Now I understand > >>> there may be other problems with this but I think we should put more > >>> though to that and provide some justification in changelogs. > >> > >>One complexity is that we have to do this early for blk-mq, since once you > >>get a request, you're already sitting on the hw tag. CoDel should actually > >>work fine at each hop, so hopefully this will as well. > > > >OK, I see. But then this suggests that any IO scheduling and / or > >cgroup-related throttling should happen before we get a request for blk-mq > >as well? And then we can still do writeback throttling below that layer? > > Not necessarily. For IO scheduling, basically we care about two parts: > > 1) Are you allowed to allocate the resources to queue some IO > 2) Are you allowed to dispatch But then it seems suboptimal to waste a relatively scarce resource (which HW tag is AFAIU) just because you happen to run from a cgroup that is bandwidth limited and thus are not allowed to dispatch? Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
