Hirokazu Takahashi wrote: > Hi, Andrea, > >>>> Ok, I will give more details of the thought process. >>>> >>>> I was thinking of maintaing an rb-tree per request queue and not an >>>> rb-tree per cgroup. This tree can contain all the bios submitted to that >>>> request queue through __make_request(). Every node in the tree will represent >>>> one cgroup and will contain a list of bios issued from the tasks from that >>>> cgroup. >>>> >>>> Every bio entering the request queue through __make_request() function >>>> first will be queued in one of the nodes in this rb-tree, depending on which >>>> cgroup that bio belongs to. >>>> >>>> Once the bios are buffered in rb-tree, we release these to underlying >>>> elevator depending on the proportionate weight of the nodes/cgroups. >>>> >>>> Some more details which I was trying to implement yesterday. >>>> >>>> There will be one bio_cgroup object per cgroup. This object will contain >>>> many bio_group objects. Each bio_group object will be created for each >>>> request queue where a bio from bio_cgroup is queued. Essentially the idea >>>> is that bios belonging to a cgroup can be on various request queues in the >>>> system. So a single object can not serve the purpose as it can not be on >>>> many rb-trees at the same time. Hence create one sub object which will keep >>>> track of bios belonging to one cgroup on a particular request queue. >>>> >>>> Each bio_group will contain a list of bios and this bio_group object will >>>> be a node in the rb-tree of request queue. For example. Lets say there are >>>> two request queues in the system q1 and q2 (lets say they belong to /dev/sda >>>> and /dev/sdb). Let say a task t1 in /cgroup/io/test1 is issueing io both >>>> for /dev/sda and /dev/sdb. >>>> >>>> bio_cgroup belonging to /cgroup/io/test1 will have two sub bio_group >>>> objects, say bio_group1 and bio_group2. bio_group1 will be in q1's rb-tree >>>> and bio_group2 will be in q2's rb-tree. bio_group1 will contain a list of >>>> bios issued by task t1 for /dev/sda and bio_group2 will contain a list of >>>> bios issued by task t1 for /dev/sdb. I thought the same can be extended >>>> for stacked devices also. >>>> >>>> I am still trying to implementing it and hopefully this is doable idea. >>>> I think at the end of the day it will be something very close to dm-ioband >>>> algorithm just that there will be no lvm driver and no notion of separate >>>> dm-ioband device. >>> Vivek, thanks for the detailed explanation. Only a comment. I guess, if >>> we don't change also the per-process optimizations/improvements made by >>> some IO scheduler, I think we can have undesirable behaviours. >>> >>> For example: CFQ uses the per-process iocontext to improve fairness >>> between *all* the processes in a system. But it doesn't have the concept >>> that there's a cgroup context on-top-of the processes. >>> >>> So, some optimizations made to guarantee fairness among processes could >>> conflict with algorithms implemented at the cgroup layer. And >>> potentially lead to undesirable behaviours. >>> >>> For example an issue I'm experiencing with my cgroup-io-throttle >>> patchset is that a cgroup can consistently increase the IO rate (always >>> respecting the max limits), simply increasing the number of IO worker >>> tasks respect to another cgroup with a lower number of IO workers. This >>> is probably due to the fact the CFQ tries to give the same amount of >>> "IO time" to all the tasks, without considering that they're organized >>> in cgroup. >> BTW this is why I proposed to use a single shared iocontext for all the >> processes running in the same cgroup. Anyway, this is not the best >> solution, because in this way all the IO requests coming from a cgroup >> will be queued to the same cfq queue. If I'm not wrong in this way we >> would implement noop (FIFO) between tasks belonging to the same cgroup >> and CFQ between cgroups. But, at least for this particular case, we >> would be able to provide fairness among cgroups. >> >> -Andrea > > I ever thought the same thing but this approach breaks the compatibility. > I think we should make ionice only effective for the processes in the > same cgroup. > > A system gives some amount of bandwidths to its cgroups, and > the processes in one of the cgroups fairly share the given bandwidth. > I think this is the straight approach. What do you think? > > I think all the CFQ-cgroup the NEC guys are working, OpenVZ team's CFQ > scheduler and dm-ioband with bio-cgroup work like this. If by "fairly share the given bandwidth" you mean "share according to their IO-nice values" then you're right on this, Hirokazu. We always use a two-level schedulers and would like to see the same behavior in anything that will be the IO-bandwidth-controller in the mainline :) > Thank you, > Hirokazu Takahashi. > > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization