Re: High load average on disk I/O on 2.6.17-rc3

[Nick Piggin]

Arjan van de Ven wrote:

> On Tue, 2006-05-09 at 11:57 +1000, Nick Piggin wrote:
>
> > Perhaps kernel threads in D state should not contribute toward load avg
>
> that would be a change from, well... a LONG time

But presently it changes all the time when we change the implementation
of pdflush or kswapd.

If we make pdflush threads blk_congestion_wait for twice as long, and
end up creating twice as many to feed the same amount of IO, our load
magically doubles but the machine is under almost exactly the same
load condition.

Back when we didn't have all these kernel threads doing work for us,
that wasn't an issue.

> The question is what "load" means; if you want to change that... then
> there are even better metrics possible. Like
> "number of processes wanting to run + number of busy spindles + number
> of busy nics + number of VM zones that are below the problem
> watermark" (where "busy" means "queue full")
>
> or 50 million other definitions. If we're going to change the meaning,
> we might as well give it a "real" meaning. 

I'm not sure if that is any better, and perhaps even worse. It does not
matter that much if VM zones are under a watermark if kswapd is taking
care of the problem and nothing ever blocks on memory IO.

(Sure kswapd will contribute to CPU usage, but that *will* be reflected
in load average)

> (And even then it is NOT a good measure for determining if the machine
> can perform more work, the graph I put in a previous mail is very real,
> and in practice it seems the saturation line is easily 4x or 5x of the
> "linear" point)

A global loadavg isn't too good anyway, as everyone has observed, there
are many independant resources. But my point is that it isn't going away
while apps still use it, so my point is that this might be an easy way to
improve it.

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