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

[Vivek Goyal <vgoyal@redhat.com>]

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.

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.

Thanks
Vivek