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


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