Re: [PATCH 04/14] blk-mq-sched: improve dispatching from sw queue

Bart Van Assche
2017-08-03T01:35:29Z

On Wed, 2017-08-02 at 11:31 +0800, Ming Lei wrote:
> On Tue, Aug 01, 2017 at 03:11:42PM +0000, Bart Van Assche wrote:
> > On Tue, 2017-08-01 at 18:50 +0800, Ming Lei wrote:
> > > On Tue, Aug 01, 2017 at 06:17:18PM +0800, Ming Lei wrote:
> > > > How can we get the accurate 'number of requests in progress' efficiently?
> >
> > Hello Ming,
> >
> > How about counting the number of bits that have been set in the tag set?
> > I am aware that these bits can be set and/or cleared concurrently with the
> > dispatch code but that count is probably a good starting point.
>
> It has to be atomic_t, which is too too heavy for us, please see the report:
>
> http://marc.info/?t=149868448400003&r=1&w=2
>
> Both Jens and I want to kill hd_struct.in_flight, but looks still no
> good way.

Hello Ming,

Sorry but I disagree that a new atomic variable should be added to keep track
of the number of busy requests. Counting the number of bits that are set in
the tag set should be good enough in this context.

Thanks,

Bart.