Re: [RFC] block/throttle: Add IO throttled information in blkcg.

[Tao Ma <tm@tao.ma>]

On 05/23/2012 04:08 AM, Vivek Goyal wrote:
> On Tue, May 22, 2012 at 11:14:55PM +0800, Tao Ma wrote:
>> On 05/22/2012 11:06 PM, Vivek Goyal wrote:
>>> On Tue, May 22, 2012 at 10:44:11PM +0800, Tao Ma wrote:
>>>> Hi Vivek,
>>>> 	Thanks for the quick response.
>>>> On 05/22/2012 07:11 PM, Vivek Goyal wrote:
>>>>> On Tue, May 22, 2012 at 04:10:36PM +0800, Tao Ma wrote:
>>>>>> From: Tao Ma <boyu.mt@taobao.com>
>>>>>>
>>>>>> Currently, if the IO is throttled by io-throttle, the SA has no idea of
>>>>>> the situation and can't report it to the real application user about
>>>>>> that he/she has to do something. So this patch adds a new interface
>>>>>> named blkio.throttle.io_throttled which indicates how many IOs are
>>>>>> currently throttled.
>>>>>
>>>>> If the only purpose is to know whether IOs are being throttled, why
>>>>> not just scan for the rules and see if respective device has any
>>>>> throttling rules or not.
>>>> Sorry, but setting a throttling rules doesn't mean the IOs are
>>>> throttled, right? So scanning doesn't work here IMHO.
>>>
>>> It means IOs will be throttled if you cross a certain rate. But yes, it
>>> does not give any information that if at time T if there are any bios
>>> throttled in the queue or not.
>>>
>>>>>
>>>>> Even if you introduce this interface, you will end up scanning for
>>>>> throttled ios against that particular device. And if IO is not happening
>>>>> at that moment or if IO rate is not exceeding the rate limit, there
>>>>> might not be any throttled ios and one might get misled.
>>>> Oh, no actually in a *clound computing* environment, it is really
>>>> useful, not misled. So let me describe it in more detail. Our product
>>>> system will limit every instance to an approximate number at first, and
>>>> then watch out the IOs being throttled. If these numbers is high, it can:
>>>> 1) Shout loudly to the application programmer about the abuse if he
>>>> sends out too much IO requests.
>>>> 2) If it is not too much and some other instances are not active, adjust
>>>> the throttled ratio so that this instance can work much faster.
>>>
>>> Ok, so you want to use this more as "congestion" parameter which tells at
>>> a given moment how busy the queue is, or in this instance how many IOs
>>> are backlogged in a cgroup due to throttling limits.
>> yeah, with this information the daemon can adjust these limits
>> automatically.
> 
> I am hoping that this daemon will monitor the file for long periods and
> will not reach to bursty traffic from application.
> 
>>>
>>> I guess, it is not a bad idea to export this stat then. Will
>>> "blkio.throttle.queued" be a better name to reflect that how many bios
>>> are currently queued in throttling layer of request queue.
>> I have thought of this name at the very first time. But there is also
>> another one named "blkio.queued" which indicated the IOs being queued in
>> the scheduler. I don't want the user to be confused and that's the
>> reason I use "blkio.throttle.io_throttled".
> 
> Actually it is blkio.io_queued which shows number of requests queued in
> CFQ in that cgroup.
> 
> CFQ and throttling are two different policies and they have separate
> files in cgroup. Ideally blkio.io_queued should have been
> blkio.cfq.io_queued but initially it did not occur to me that I should
> qualify these files with policy name.
> 
> Later when throttling policy came along, then I qualified new files with
> policy name. blkio.throttle.*.
> 
> In summary, blkio.io_queued gives stats of io queued at CFQ level. So it
> makes sense to create blkio.throttle.io_queued which tells how many
> bios are currently throttled and queued in throttling layer in this
> request queue from this cgroup.
OK, I am fine with any name actually. ;) I will use it in the v2.

Thanks
Tao
> 
> Thanks
> Vivek