[Devel] Re: RFC: I/O bandwidth controller

[Andrea Righi]

Hirokazu Takahashi wrote:
>>>>>> 3. & 4. & 5. - I/O bandwidth shaping & General design aspects
>>>>>>
>>>>>> The implementation of an I/O scheduling algorithm is to a certain extent
>>>>>> influenced by what we are trying to achieve in terms of I/O bandwidth
>>>>>> shaping, but, as discussed below, the required accuracy can determine
>>>>>> the layer where the I/O controller has to reside. Off the top of my
>>>>>> head, there are three basic operations we may want perform:
>>>>>>   - I/O nice prioritization: ionice-like approach.
>>>>>>   - Proportional bandwidth scheduling: each process/group of processes
>>>>>> has a weight that determines the share of bandwidth they receive.
>>>>>>   - I/O limiting: set an upper limit to the bandwidth a group of tasks
>>>>>> can use.
>>>>> Use a deadline-based IO scheduling could be an interesting path to be
>>>>> explored as well, IMHO, to try to guarantee per-cgroup minimum bandwidth
>>>>> requirements.
>>>> Please note that the only thing we can do is to guarantee minimum
>>>> bandwidth requirement when there is contention for an IO resource, which
>>>> is precisely what a proportional bandwidth scheduler does. An I missing
>>>> something?
>>> Correct. Proportional bandwidth automatically allows to guarantee min
>>> requirements (instead of IO limiting approach, that needs additional
>>> mechanisms to achive this).
>>>
>>> In any case there's no guarantee for a cgroup/application to sustain
>>> i.e. 10MB/s on a certain device, but this is a hard problem anyway, and
>>> the best we can do is to try to satisfy "soft" constraints.
>> I think guaranteeing the minimum I/O bandwidth is very important. In the 
>> business site, especially in streaming service system, administrator requires 
>> the functionality to satisfy QoS or performance of their service. 
>> Of course, IO throttling is important, but, personally, I think guaranteeing 
>> the minimum bandwidth is more important than limitation of maximum bandwidth 
>> to satisfy the requirement in real business sites.
>> And I know Andrea’s io-throttle patch supports the latter case well and it is 
>> very stable. 
>> But, the first case(guarantee the minimum bandwidth) is not supported in any 
>> patches.
>> Is there any plans to support it? and Is there any problems in implementing it?
>> I think if IO controller can support guaranteeing the minimum bandwidth and 
>> work-conserving mode simultaneously, it more easily satisfies the requirement 
>> of the business sites.
>> Additionally, I didn’t understand “Proportional bandwidth automatically allows 
>> to guarantee min
>> requirements” and “soft constraints”.
>> Can you give me a advice about this ? 
>> Thanks in advance.
>>
>> Dong-Jae Kang
> 
> I think this is what dm-ioband does.
> 
> Let's say you make two groups share the same disk, and give them
> 70% of the bandwidth the disk physically has and 30% respectively.
> This means the former group is almost guaranteed to be able to use
> 70% of the bandwidth even when the latter one is issuing quite
> a lot of I/O requests.
> 
> Yes, I know there exist head seek lags with traditional magnetic disks,
> so it's important to improve the algorithm to reduce this overhead.
> 
> And I think it is also possible to add a new scheduling policy to
> guarantee the minimum bandwidth. It might be cool if some group can
> use guranteed bandwidths and the other share the rest on proportional
> bandwidth policy.
> 
> Thanks,
> Hirokazu Takahashi.

With IO limiting approach minimum requirements are supposed to be
guaranteed if the user configures a generic block device so that the sum
of the limits doesn't exceed the total IO bandwidth of that device. But,
in principle, there's nothing in "throttling" that guarantees "fairness"
among different cgroups doing IO on the same block devices, that means
there's nothing to guarantee minimum requirements (and this is the
reason because I liked the Satoshi's CFQ-cgroup approach together with
io-throttle).

A more complicated issue is how to evaluate the total IO bandwidth of a
generic device. We can use some kind of averaging/prediction, but
basically it would be inaccurate due to the mechanic of disks (head
seeks, but also caching, buffering mechanisms implemented directly into
the device, etc.). It's a hard problem. And the same problem exists also
for proportional bandwidth as well, in terms of IO rate predictability I
mean.

The only difference is that with proportional bandwidth you know that
(taking the same example reported by Hirokazu) with i.e. 10 similar IO
requests, 7 will be dispatched to the first cgroup and 3 to the other
cgroup. So, you don't need anything to guarantee "fairness", but it's
hard also for this case to evaluate the cost of the 7 IO requests
respect to the cost of the other 3 IO requests as seen by user
applications, that is the cost the users care about.

-Andrea
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