Date: Wed, 15 Nov 2000 17:07:02 -0200 (BRDT) From: Rik van Riel <riel@conectiva.com.br> To: Terry Lambert <tlambert@primenet.com> Cc: void <float@firedrake.org>, hackers@FreeBSD.ORG Subject: Re: "iowait" CPU state Message-ID: <Pine.LNX.4.21.0011151704110.5584-100000@duckman.distro.conectiva> In-Reply-To: <200011151730.KAA12666@usr01.primenet.com>
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On Wed, 15 Nov 2000, Terry Lambert wrote:
> > > > Thank you! This gets the me disk %busy, which is one of the things I
> > > > was looking for. Now, can anyone tell me how to tell what percentage of
> > > > processor time is being spent waiting for disk I/O to complete?
> > >
> > > Uh, none?
> > >
> > > If there is disk I/O pending, the processor just runs a
> > > different process... am I missing your question?
> >
> > I guess it might be useful to see the difference between
> > "true" idle time and time the system couldn't do anything
> > useful because it was blocked on the disk (but /should/
> > have done something useful...).
>
> You mean because the programmer didn't interleave their I/O,
> and wrote to a threads interface, or some other interface
> that's prone to subsystem stalling, instead?
Interleaving IO only makes sense when you have tons of
parallelisable jobs. If you have one big serial job this
doesn't buy you anything...
Yes, you can use a separate thread to queue IO in
advance, but in this case it might just be useful to
have the %iowait statistic so you know how much work
to queue in advance.
Then again, this may be a bad example. I can't quite
put my finger on it, but somehow I have the idea that
the %iowait may be a useful statistic to keep...
> Modern bloat-ware really pisses me off; I built the bind
> library the other day: the frigging thing was 4M, unstripped.
How does this affect the (non?-)usefullness of the
%iowait statistic?
regards,
Rik
--
"What you're running that piece of shit Gnome?!?!"
-- Miguel de Icaza, UKUUG 2000
http://www.conectiva.com/ http://www.surriel.com/
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