Hi Vivek, From: Vivek Goyal <vgoyal@xxxxxxxxxx> Subject: Re: [patch 0/4] [RFC] Another proportional weight IO controller Date: Tue, 25 Nov 2008 11:27:20 -0500 > On Tue, Nov 25, 2008 at 11:33:59AM +0900, Ryo Tsuruta wrote: > > Hi Vivek, > > > > > > > Ryo, do you still want to stick to two level scheduling? Given the problem > > > > > of it breaking down underlying scheduler's assumptions, probably it makes > > > > > more sense to the IO control at each individual IO scheduler. > > > > > > > > I don't want to stick to it. I'm considering implementing dm-ioband's > > > > algorithm into the block I/O layer experimentally. > > > > > > Thanks Ryo. Implementing a control at block layer sounds like another > > > 2 level scheduling. We will still have the issue of breaking underlying > > > CFQ and other schedulers. How to plan to resolve that conflict. > > > > I think there is no conflict against I/O schedulers. > > Could you expain to me about the conflict? > > Because we do the buffering at higher level scheduler and mostly release > the buffered bios in the FIFO order, it might break the underlying IO > schedulers. Generally it is the decision of IO scheduler to determine in > what order to release buffered bios. > > For example, If there is one task of io priority 0 in a cgroup and rest of > the tasks are of io prio 7. All the tasks belong to best effort class. If > tasks of lower priority (7) do lot of IO, then due to buffering there is > a chance that IO from lower prio tasks is seen by CFQ first and io from > higher prio task is not seen by cfq for quite some time hence that task > not getting it fair share with in the cgroup. Similiar situations can > arise with RT tasks also. Thanks for your explanation. I think that the same thing occurs without the higher level scheduler, because all the tasks issuing I/Os are blocked while the underlying device's request queue is full before those I/Os are sent to the I/O scheduler. > > > What do you think about the solution at IO scheduler level (like BFQ) or > > > may be little above that where one can try some code sharing among IO > > > schedulers? > > > > I would like to support any type of block device even if I/Os issued > > to the underlying device doesn't go through IO scheduler. Dm-ioband > > can be made use of for the devices such as loop device. > > > > What do you mean by that IO issued to underlying device does not go > through IO scheduler? loop device will be associated with a file and > IO will ultimately go to the IO scheduler which is serving those file > blocks? How about if the files is on an NFS-mounted file system? > What's the use case scenario of doing IO control at loop device? > Ultimately the resource contention will take place on actual underlying > physical device where the file blocks are. Will doing the resource control > there not solve the issue for you? I don't come up with any use case, but I would like to make the resource controller more flexible. Actually, a certain block device that I'm using does not use the I/O scheduler. Thanks, Ryo Tsuruta _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
