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

[Tao Ma <tm@tao.ma>]

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 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".

Thanks
Tao