[dm-devel] Re: [PATCH 02/20] io-controller: Common flat fair queuing code in elevaotor layer

[Vivek Goyal]

On Tue, Jun 30, 2009 at 02:40:36PM +0800, Gui Jianfeng wrote:
> Vivek Goyal wrote:
> ...
> > +
> > +/*
> > + * Do the accounting. Determine how much service (in terms of time slices)
> > + * current queue used and adjust the start, finish time of queue and vtime
> > + * of the tree accordingly.
> > + *
> > + * Determining the service used in terms of time is tricky in certain
> > + * situations. Especially when underlying device supports command queuing
> > + * and requests from multiple queues can be there at same time, then it
> > + * is not clear which queue consumed how much of disk time.
> > + *
> > + * To mitigate this problem, cfq starts the time slice of the queue only
> > + * after first request from the queue has completed. This does not work
> > + * very well if we expire the queue before we wait for first and more
> > + * request to finish from the queue. For seeky queues, we will expire the
> > + * queue after dispatching few requests without waiting and start dispatching
> > + * from next queue.
> > + *
> > + * Not sure how to determine the time consumed by queue in such scenarios.
> > + * Currently as a crude approximation, we are charging 25% of time slice
> > + * for such cases. A better mechanism is needed for accurate accounting.
> > + */
> 
>   Hi Vivek,
> 
>   The comment is out of date, would you update it accordingly?
> 

Thanks Gui. Yes, I will update it in next posting.

Vivek