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

    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.


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