Messages in this thread | | | Date | Thu, 02 Sep 2010 08:30:37 +0800 | From | Gui Jianfeng <> | Subject | Re: [RFC] [PATCH] cfq-iosched: add cfq group hierarchical scheduling support |
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Nauman Rafique wrote: > On Wed, Sep 1, 2010 at 10:10 AM, Vivek Goyal <vgoyal@redhat.com> wrote: >> On Wed, Sep 01, 2010 at 08:49:26AM -0700, Nauman Rafique wrote: >>> On Wed, Sep 1, 2010 at 1:50 AM, Gui Jianfeng <guijianfeng@cn.fujitsu.com> wrote: >>>> Vivek Goyal wrote: >>>>> On Tue, Aug 31, 2010 at 08:40:19AM -0700, Nauman Rafique wrote: >>>>>> On Tue, Aug 31, 2010 at 5:57 AM, Vivek Goyal <vgoyal@redhat.com> wrote: >>>>>>> On Tue, Aug 31, 2010 at 08:29:20AM +0800, Gui Jianfeng wrote: >>>>>>>> Vivek Goyal wrote: >>>>>>>>> On Mon, Aug 30, 2010 at 02:50:40PM +0800, Gui Jianfeng wrote: >>>>>>>>>> Hi All, >>>>>>>>>> >>>>>>>>>> This patch enables cfq group hierarchical scheduling. >>>>>>>>>> >>>>>>>>>> With this patch, you can create a cgroup directory deeper than level 1. >>>>>>>>>> Now, I/O Bandwidth is distributed in a hierarchy way. For example: >>>>>>>>>> We create cgroup directories as following(the number represents weight): >>>>>>>>>> >>>>>>>>>> Root grp >>>>>>>>>> / \ >>>>>>>>>> grp_1(100) grp_2(400) >>>>>>>>>> / \ >>>>>>>>>> grp_3(200) grp_4(300) >>>>>>>>>> >>>>>>>>>> If grp_2 grp_3 and grp_4 are contending for I/O Bandwidth, >>>>>>>>>> grp_2 will share 80% of total bandwidth. >>>>>>>>>> For sub_groups, grp_3 shares 8%(20% * 40%), grp_4 shares 12%(20% * 60%) >>>>>>>>>> >>>>>>>>>> Design: >>>>>>>>>> o Each cfq group has its own group service tree. >>>>>>>>>> o Each cfq group contains a "group schedule entity" (gse) that >>>>>>>>>> schedules on parent cfq group's service tree. >>>>>>>>>> o Each cfq group contains a "queue schedule entity"(qse), it >>>>>>>>>> represents all cfqqs located on this cfq group. It schedules >>>>>>>>>> on this group's service tree. For the time being, root group >>>>>>>>>> qse's weight is 1000, and subgroup qse's weight is 500. >>>>>>>>>> o All gses and qse which belones to a same cfq group schedules >>>>>>>>>> on the same group service tree. >>>>>>>>> Hi Gui, >>>>>>>>> >>>>>>>>> Thanks for the patch. I have few questions. >>>>>>>>> >>>>>>>>> - So how does the hierarchy look like, w.r.t root group. Something as >>>>>>>>> follows? >>>>>>>>> >>>>>>>>> >>>>>>>>> root >>>>>>>>> / | \ >>>>>>>>> q1 q2 G1 >>>>>>>>> >>>>>>>>> Assume there are two processes doin IO in root group and q1 and q2 are >>>>>>>>> cfqq queues for those processes and G1 is the cgroup created by user. >>>>>>>>> >>>>>>>>> If yes, then what algorithm do you use to do scheduling between q1, q2 >>>>>>>>> and G1? IOW, currently we have two algorithms operating in CFQ. One for >>>>>>>>> cfqq and other for groups. Group algorithm does not use the logic of >>>>>>>>> cfq_slice_offset(). >>>>>>>> Hi Vivek, >>>>>>>> >>>>>>>> This patch doesn't break the original sheduling logic. That is cfqg => st => cfqq. >>>>>>>> If q1 and q2 in root group, I treat q1 and q2 bundle as a queue sched entity, and >>>>>>>> it will schedule on root group service with G1, as following: >>>>>>>> >>>>>>>> root group >>>>>>>> / \ >>>>>>>> qse(q1,q2) gse(G1) >>>>>>>> >>>>>>> Ok. That's interesting. That raises another question that how hierarchy >>>>>>> should look like. IOW, how queue and groups should be treated in >>>>>>> hierarchy. >>>>>>> >>>>>>> CFS cpu scheduler treats queues and group at the same level. That is as >>>>>>> follows. >>>>>>> >>>>>>> root >>>>>>> / | \ >>>>>>> q1 q2 G1 >>>>>>> >>>>>>> In the past I had raised this question and Jens and corrado liked treating >>>>>>> queues and group at same level. >>>>>>> >>>>>>> Logically, q1, q2 and G1 are all children of root, so it makes sense to >>>>>>> treat them at same level and not group q1 and q2 in to a single entity and >>>>>>> group. >>>>>>> >>>>>>> One of the possible way forward could be this. >>>>>>> >>>>>>> - Treat queue and group at same level (like CFS) >>>>>>> >>>>>>> - Get rid of cfq_slice_offset() logic. That means without idling on, there >>>>>>> will be no ioprio difference between cfq queues. I think anyway as of >>>>>>> today that logic helps in so little situations that I would not mind >>>>>>> getting rid of it. Just that Jens should agree to it. >>>>>>> >>>>>>> - With this new scheme, it will break the existing semantics of root group >>>>>>> being at same level as child groups. To avoid that, we can probably >>>>>>> implement two modes (flat and hierarchical), something similar to what >>>>>>> memory cgroup controller has done. May be one tunable in root cgroup of >>>>>>> blkio "use_hierarchy". By default everything will be in flat mode and >>>>>>> if user wants hiearchical control, he needs to set user_hierarchy in >>>>>>> root group. >>>>>> Vivek, may be I am reading you wrong here. But you are first >>>>>> suggesting to add more complexity to treat queues and group at the >>>>>> same level. Then you are suggesting add even more complexity to fix >>>>>> the problems caused by that approach. >>>>>> >>>>>> Why do we need to treat queues and group at the same level? "CFS does >>>>>> it" is not a good argument. >>>>> Sure it is not a very good argument but at the same time one would need >>>>> a very good argument that why we should do things differently. >>>>> >>>>> - If a user has mounted cpu and blkio controller together and both the >>>>> controllers are viewing the same hierarchy differently, then it is >>>>> odd. We need a good reason that why different arrangement makes sense. >>>> Hi Vivek, >>>> >>>> Even if we mount cpu and blkio together, to me, it's ok for cpu and blkio >>>> having their own logic, since they are totally different cgroup subsystems. >>>> >>>>> - To me, both group and cfq queue are children of root group and it >>>>> makes sense to treat them independent childrens instead of putting >>>>> all the queues in one logical group which inherits the weight of >>>>> parent. >>>>> >>>>> - With this new scheme, I am finding it hard to visualize the hierachy. >>>>> How do you assign the weights to queue entities of a group. It is more >>>>> like a invisible group with-in group. We shall have to create new >>>>> tunable which can speicy the weight for this hidden group. >>>> For the time being, the root "qse" weight is 1000 and others is 500, they don't >>>> inherit the weight of parent. I was thinking that maybe we can determine the qse >>>> weight in term of the queue number and weight in this group and subgroups. >>>> >>>> Thanks, >>>> Gui >>>> >>>>> >>>>> So in summary I am liking the "queue at same level as group" scheme for >>>>> two reasons. >>>>> >>>>> - It is more intutive to visualize and implement. It follows the true >>>>> hierarchy as seen by cgroup file system. >>>>> >>>>> - CFS has already implemented this scheme. So we need a strong arguemnt >>>>> to justify why we should not follow the same thing. Especially for >>>>> the case where user has co-mounted cpu and blkio controller. >>>>> >>>>> - It can achieve the same goal as "hidden group" proposal just by >>>>> creating a cgroup explicitly and moving all threads in that group. >>>>> >>>>> Why do you think that "hidden group" proposal is better than "treating >>>>> queue at same level as group" ? >>> There are multiple reasons for "hidden group" proposal being a better approach. >>> >>> - "Hidden group" would allow us to keep scheduling queues using the >>> CFQ queue scheduling logic. And does not require any major changes in >>> CFQ. Aren't we already using that approach to deal with queues at the >>> root group? >> Currently we are operating in flat mode where all the groups are at >> same level (irrespective their position in cgroup hiearchy). >> >>> - If queues and groups are treated at the same level, queues can end >>> up in root cgroup. And we cannot put an upper bound on the number of >>> those queues. Those queues can consume system resources in proportion >>> to their number, causing the performance of groups to suffer. If we >>> have "hidden group", we can configure it to a small weight, and that >>> would limit the impact these queues in root group can have. >> To limit the impact of other queues in cgroup, one can use libcgroup to >> automatically place new threads or tasks into a subgroup. >> >> I understand that kernel doing it by default should help though. It is >> less work in terms of configuration. But I am not sure that's a good >> argument to design kernel functionality. Kernel functionality should be >> pretty generic. >> >> Anyway, how would you assign the weight to the hidden group. What's the >> interface for that? A new cgroup file inside each cgroup? Personally >> I think that's little odd interface. Every group has one hidden group >> where all the queues in that group go and weight of that group can be >> specified by a cgroup file. > > I think picking a reasonable default weight at compile time is not > that bad an option, given that threads showing up in the "hidden > group" is an uncommon case.
Hi Nauman,
Later, I think we might adjust the weight of "hidden group" automatically according to the queue number and subgroup number and their weight. But for the time being, i'd choose a fixed value for the sake of simplicity.
Gui
> >> But anyway, I am not tied to any of the approach. I am just trying to >> make sure that we have put enough thought into it as changing it later >> will be hard. >> >> Vivek >> > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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