Re: [PATCH] cfq-iosched: replace sync_flight by rq_in_driver[BLK_RW_SYNC]

[Corrado Zoccolo <czoccolo@gmail.com>]

On Wed, Dec 30, 2009 at 7:47 PM, Jens Axboe <jens.axboe@oracle.com> wrote:
> On Wed, Dec 30 2009, Corrado Zoccolo wrote:
>> According to my intuition (and brief testing), sync_flight is always
>> equal to rq_in_driver[BLK_RW_SYNC] at the point of usage, so it can
>> be removed and replaced by the other.
>
> They are not fully identical. ->sync_flight is incremented on insertion
> on the dispatch list, ->rq_in_driver not until the request is activated
> (eg the driver has retrieved it and wants to dispatch to the hardware).
Usually (only exceptions are forced dispatch, or when a conflict in the
rb tree is found), a request is activated as soon as cfq returns from
cfq_dispatch_requests.

> They will usually be identical, but that may not be true for requeues
> for instance.

For our purpose, it is sufficient that in cfq_may_dispatch, they are either
both 0 or both non-0.
Since we have sync_flight >= rq_in_driver[1], the only question is:
can the number of requests in the driver drop to 0 with requests still
in flight?

I'm asking because to drain async requests, we are using the rq_in_driver
counter instead. Maybe they need the same treatment.

Thanks
Corrado

>
> --
> Jens Axboe
>
>