[Devel] Re: dm-ioband + bio-cgroup benchmarks

[Pavel Emelyanov]

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.
> 
> 

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