lkml.org 
[lkml]   [2014]   [Jul]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: scsi-mq V2
    On 2014-07-10 15:50, Benjamin LaHaise wrote:
    > On Thu, Jul 10, 2014 at 03:48:10PM +0200, Jens Axboe wrote:
    >> On 2014-07-10 15:44, Benjamin LaHaise wrote:
    >>> On Thu, Jul 10, 2014 at 03:39:57PM +0200, Jens Axboe wrote:
    >>>> That's how fio always runs, it sets up the context with the exact queue
    >>>> depth that it needs. Do we have a good enough understanding of other aio
    >>>> use cases to say that this isn't the norm? I would expect it to be, it's
    >>>> the way that the API would most obviously be used.
    >>>
    >>> The problem with this approach is that it works very poorly with per cpu
    >>> reference counting's batching of references, which is pretty much a
    >>> requirement now that many core systems are the norm. Allocating the bare
    >>> minimum is not the right thing to do today. That said, the default limits
    >>> on the number of requests probably needs to be raised.
    >>
    >> Sorry, that's a complete cop-out. Then you handle this internally,
    >> allocate a bigger pool and cap the limit if you need to. Look at the
    >> API. You pass in the number of requests you will use. Do you expect
    >> anyone to double up, just in case? Will never happen.
    >>
    >> But all of this is side stepping the point that there's a real bug
    >> reported here. The above could potentially explain the "it's using X
    >> more CPU, or it's Y slower". The above is a softlock, it never completes.
    >
    > I'm not trying to cop out on this -- I'm asking for a data point to see
    > if changing the request limits has any effect.

    Fair enough, if the question is "does it solve the regression", then
    it's a valid data point. Rob/Doug, for fio, you can just double the
    iodepth passed in in engines/libaio:fio_libaio_init() and test with that
    and see if it makes a difference.

    --
    Jens Axboe



    \
     
     \ /
      Last update: 2014-07-10 16:41    [W:4.188 / U:0.264 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site