On Fri, May 03, 2013 at 12:14:18PM -0700, Tejun Heo wrote: > On Fri, May 03, 2013 at 03:08:23PM -0400, Vivek Goyal wrote: > > T1 T2 T3 T4 T5 T6 T7 > > parent: b1 b2 b3 b4 b5 > > child: b1 b2 b3 b4 b5 > > > > > > So continuity breaks down because application is waiting for previous > > IO to finish. This forces expiry of existing time slices and new time > > slice start both in child and parent and penalty keep on increasing. > > It's a problem even in flat mode as the "child" above can easily be > just a process which is throttling itself and it won't be able to get > the configured bandwidth due to the scheduling bubbles introduced > whenever new slice is started. Shouldn't be too difficult to get rid > of, right?
Key thing here is when to start a new slice. Generally when an IO has been dispatched from a group, we do not expire slice immediately. We kind of give group some grace period of throtl_slice (100ms). If next IO does not come with-in that duration, we start a fresh slice upon next IO arrival. I think similar problem should happen if there are two stacked devices and both are doing throttling and if delays between 2 IOs are big enough that it forces expirty of slice on each device. Atleast for the hiearchy case, we should be able to start a fresh time slice when child transfer bio to parent. I will write a patch and do some experiment. Thanks Vivek -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
