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    SubjectRe: [RFC] block/throttle: Add IO throttled information in blkcg.
    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 <>
    > >>>>
    > >>>> 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.


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