Re: [RFC v1] add new io-scheduler to use cgroup on high-speed device

[sanbai <sanbai@taobao.com>]

On 2013年06月08日 03:53, Vivek Goyal wrote:
> On Fri, Jun 07, 2013 at 11:09:54AM +0800, sanbai wrote:
>> On 2013年06月05日 21:30, Vivek Goyal wrote:
>>> On Wed, Jun 05, 2013 at 10:09:31AM +0800, Robin Dong wrote:
>>>> We want to use blkio.cgroup on high-speed device (like fusionio) for our mysql clusters.
>>>> After testing different io-scheduler, we found that  cfq is too slow and deadline can't run on cgroup.
>>> So why not enhance deadline to be able to be used with cgroups instead of
>>> coming up with a new scheduler?
>> I think if we add cgroups support into deadline, it will not be
>> suitable to call "deadline" anymore...so a new ioscheduler and a new
>> name may not confuse users.
> Nobody got confused when we added cgroup support to CFQ. Not that
> I am saying go add support to deadline. I am just saying that need
> for cgroup support does not sound like it justfies need of a new
> IO scheduler.
>
> [..]
>>> Can you give more details. Do you idle? Idling kills performance. If not,
>>> then without idling how do you achieve performance differentiation.
>> We don't idle, when comes to .elevator_dispatch_fn，we just compute
>> quota for every group:
>>
>> quota = nr_requests - rq_in_driver;
>> group_quota = quota * group_weight / total_weight;
>>
>> and dispatch 'group_quota' requests for the coordinate group.
>> Therefore high-weight group
>> will dispatch more requests than low-weight group.
> Ok, this works only if all the groups are full all the time otherwise
> groups will lose their fair share. This simplifies the things a lot.
> That is fairness is provided only if group is always backlogged. In
> practice, this happens only if a group is doing IO at very high rate
> (like your fio scripts). Have you tried running any real life workload
> in these cgroups (apache, databases etc) and see how good is service
> differentiation.
>
> Anyway, sounds like this can be done at generic block layer like
> blk-throtl and it can sit on top so that it can work with all schedulers
> and can also work with bio based block drivers.
That's a new idea, I will give a try later.
>    
>
> [..]
>> I do the test again for cfq (slice_idle=0, quatum=128) and tpps
>>
>> cfq (slice_idle=0, quatum=128)
>> groupname iops avg-rt(ms) max-rt(ms)
>> test1 16148 15 188
>> test2 12756 20 117
>> test3 9778 26 268
>> test4 6198 41 209
>>
>> tpps
>> groupname iops avg-rt(ms) max-rt(ms)
>> test1 17292 14 65
>> test2 15221 16 80
>> test3 12080 21 66
>> test4 7995 32 90
>>
>> Looks cfq with is much better than before.
> Yep, I am sure there are more simple opportunites for optimization
> where it can help. Can you try couple more things.
>
> - Drive even deeper queue depth. Set quantum=512.
>
> - set group_idle=0.
I changed the iodepth to 512 in fio script and the new result is:

cfq (group_idle=0, quantum=512)
groupname    iops        avg-rt(ms)   max-rt(ms)
test1               15259    33                305
test2               11858    42                345
test3               8885      57                335
test4               5738      89                355

cfq (group_idle=0, quantum=512, slice_sync=10)
groupname    iops        avg-rt(ms)   max-rt(ms)
test1               16507    31                177
test2               12896    39                366
test3               9301      55                188
test4               6023      84                545

tpps
groupname    iops        avg-rt(ms)   max-rt(ms)
test1               16316    31                99
test2               15066    33                106
test3               12182    42                101
test4               8350      61                180

looks cfq works much better now.
>
>    Ideally this should effectively emulate what you are doing. That is try
>    to provide fairness without idling on group.
>
>    In practice I could not keep group queue full and before group exhausted
>    its slice, it got empty and got deleted from service tree and lost its
>    fair share. So if group_idle=0 leads to no service differentiation,
>    try slice_sync=10 and see what happens.
>
> Thanks
> Vivek


-- 

Robin Dong
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阿里巴巴 集团 核心系统部 内核组
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email：sanbai@taobao.com