Re: IOPS based scheduler (Was: Re: [PATCH 18/21] blkcg: move blkio_group_conf->weightto cfq)

[Tao Ma]

On 04/04/2012 09:37 PM, Vivek Goyal wrote:
> On Wed, Apr 04, 2012 at 05:35:49AM -0700, Shaohua Li wrote:
> 
> [..]
>>>> How iops_weight and switching different than CFQ group scheduling logic?
>>>> I think shaohua was talking of using similar logic. What would you do
>>>> fundamentally different so that without idling you will get service
>>>> differentiation?
>>> I am thinking of differentiate different groups with iops, so if there
>>> are 3 groups(the weight are 100, 200, 300) we can let them submit 1 io,
>>> 2 io and 3 io in a round-robin way. With a intel ssd, every io can be
>>> finished within 100us. So the maximum latency for one io is about 600us,
>>> still less than 1ms. But with cfq, if all the cgroups are busy, we have
>>> to switch between these group in ms which means the maximum latency will
>>> be 6ms. It is terrible for some applications since they use ssds now.
>> Yes, with iops based scheduling, we do queue switching for every request.
>> Doing the same thing between groups is quite straightforward. The only issue
>> I found is this will introduce more process context switch, this isn't
>> a big issue
>> for io bound application, but depends. It cuts latency a lot, which I
>> guess is more
>> important for web 2.0 application.
> 
> In iops_mode(), expire each cfqq after dispatch of 1 or bunch of requests
> and you should get the same behavior (with slice_idle=0 and group_idle=0).
> So why write a new scheduler.
really? How could we config cfq to work like this? Or you mean we can
change the code for it?
> 
> Only thing is that with above, current code will provide iops fairness only
> for groups. We should be able to tweak queue scheduling to support iops
> fairness also.
OK, as I have said in another e-mail another my concern is the
complexity. It will make cfq too much complicated. I just checked the
source code of shaohua's original patch, fiops scheduler is only ~700
lines, so with cgroup support added it would be ~1000 lines I guess.
Currently cfq-iosched.c is around ~4000 lines even after Tejun's cleanup
of io context...

Thanks
Tao
> 
> Anyway, we will end up doing that at some point of time. Supporting two
> scheduling algorihtms for queue and groups is not sustainable. There are
> already calls to make CFQ hierarchical and in that case both queue and
> groups need to be on a single service tree and that means need to follow
> same algorithm for scheduling.
> 
> Thanks
> Vivek
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