Subject: Re: Throttling IO Requests in NetBSD via Congestion Control
To: TlorD <>
From: Bill Studenmund <>
List: tech-kern
Date: 08/28/2006 10:23:02
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On Mon, Aug 28, 2006 at 02:21:22PM +0200, TlorD wrote:
> Bill Studenmund wrote:
> > On Mon, Aug 21, 2006 at 04:46:59PM -0400, Kurt J. Lidl wrote:
> >> On Mon, Aug 21, 2006 at 01:35:03PM -0700, Bill Studenmund wrote:
> >> As for the slowdown that is seen, you might just be forcing more
> >> head I/O on the platter, which will quickly dominate any other sources
> >> of slowness.
> >=20
> > I don't think so. I can certainly see extra seek action contributing to=
> > reads not catching up as much as writes are restrained (i.e. I could se=
> > this contributing to a 20% slowdown in writes turning into less than a =
> > increase in reads). But we're seeing fewer reads overall.
> Just a random thought: what if the induced sleep is interfering with the
> I/O scheduler/disk buffers, thus causing more seeks?

Turns out it's not. It's an issue with the page daemon not flushing pages=
fast enough.

> Let me try to clarify: good reads and writes are big, sequential ones,
> sent one after the other to the disk which then moves the head as little
> as possible. If, by adding sleeps, writes are sent few per batch, they
> can't be coalesced well and will thus be transformed into bad (small)
> writes, which would skyrocket the seek number thus lowering the read
> rate. I think.

I don't think the effect will be so bad as to skyrocket the write seeks.=20
The _count_ over time may well go up, but the whole idea is that they are=
spread out in time.

Take care,


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