Re: [1/1] Block device throttling [Re: Distributed storage.]

[Daniel Phillips <phillips@phunq.net>]

On Tuesday 28 August 2007 02:35, Evgeniy Polyakov wrote:
> On Mon, Aug 27, 2007 at 02:57:37PM -0700, Daniel Phillips 
(phillips@phunq.net) wrote:
> > Say Evgeniy, something I was curious about but forgot to ask you
> > earlier...
> >
> > On Wednesday 08 August 2007 03:17, Evgeniy Polyakov wrote:
> > > ...All oerations are not atomic, since we do not care about
> > > precise number of bios, but a fact, that we are close or close
> > > enough to the limit.
> > > ... in bio->endio
> > > +			q->bio_queued--;
> >
> > In your proposed patch, what prevents the race:
> >
> > 			cpu1						cpu2
> >
> > 	read q->bio_queued
> > 									q->bio_queued--
> > 	write q->bio_queued - 1
> > 	Whoops! We leaked a throttle count.
>
> We do not care about one cpu being able to increase its counter
> higher than the limit, such inaccuracy (maximum bios in flight thus
> can be more than limit, difference is equal to the number of CPUs -
> 1) is a price for removing atomic operation. I thought I pointed it
> in the original description, but might forget, that if it will be an
> issue, that atomic operations can be introduced there. Any
> uber-precise measurements in the case when we are close to the edge
> will not give us any benefit at all, since were are already in the
> grey area.

This is not just inaccurate, it is suicide.  Keep leaking throttle 
counts and eventually all of them will be gone.  No more IO
on that block device!

> Another possibility is to create a queue/device pointer in the bio
> structure to hold original device and then in its backing dev
> structure add a callback to recalculate the limit, but it increases
> the size of the bio. Do we need this?

Different issue.  Yes, I think we need a nice simple approach like
that, and prove it is stable before worrying about the size cost.

Regards,

Daniel
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