Subject: Re: Throttling IO Requests in NetBSD via Congestion Control
To: Matt Thomas <>
From: Bill Studenmund <>
List: tech-kern
Date: 08/21/2006 21:15:31
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On Mon, Aug 21, 2006 at 04:21:27PM -0700, Matt Thomas wrote:
> Bill Studenmund wrote:
> >On Mon, Aug 21, 2006 at 04:46:50PM -0400, Thor Lancelot Simon wrote:
> >>
> >>I cannot imagine how, in general, doing that could actually have any ot=
> >>than a negative performance impact.
> >
> >Depends.
> >
> >If our congestion prediction model is accurate, then we can predict=20
> >congestion before we encounter it. Thus for a correct model, I believe=
> >that there is _a_ value for _X_ that will work well.
> I disagree.  X will vary on the "quality" of writes.  If you have lots of
> sequential writes, the amount of them you can do will be higher than if y=
> lots of random writes.  So the threshold will vary.

Please feel free to dive in and help extend the algorithm to handle this.=
You are right, but we have to start somewhere; it does little good to=20
argue about paint color before you've framed the walls. :-)

As a minor asside, I'd prefer keeping _X_ the same and just making=20
"poorer" writes cost more, with all costs being compared against the=20
constant _X_.

One problem, of course, is detecting sequential vs random disk i/o at the=
levels where we make these measurements.

Take care,


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