Re: Page aging broken in 2.6

[Roger Luethi <rl@hellgate.ch>]

On Sun, 28 Dec 2003 08:35:28 -0800, William Lee Irwin III wrote:
> > the aggregated reference frequency is all that matters. I was merely
> > pointing out how the number of processes referencing a page could affect
> > performance as well. Reference frequency is used as an estimator for
> > the _likelihood_ of a fault in the future, but the potential _impact_
> > of a fault grows with the number of processes that may block on it.
> > It is one possible (though not necessarily the most likely) explanation
> > for the symptoms I see with 2.6.
> 
> I guess caution against LFU is uncontroversial.

My bad. What I said is true for both LRU and LFU (they try to predict
the probability of future references), but I wrote "frequency" because
that happened to be on my mind (for unrelated reasons). The point was
basically: risk = probability * damage

> I'm not convinced what vmstat gets out of 2.4 is entirely comparable to
> what it gets out of 2.6. "blocked" and "running" are collected very

Agreed. OTOH those readings are consistent with other observations I
made. It should even be possible to add up the reported idle times and
receive a ballpark figure for the slowdown compared to a system with
more than enough memory.

> differently in 2.6. iowait shouldn't be collected on 2.4 at all.

True. If 2.4 reports idle time during a compile benchmark, though, it
seems plausible to assume it is IO wait. And if 2.6 takes much longer
than 2.4 to complete, it is due to time spend waiting for I/O (minus
some difference in system overhead) -- the work done in user space is
equal, after all.

> This could probably be addressed by backporting 2.6's reporting methods
> to 2.4 so the two kernels use similar reporting mechanisms.

I don't think it's worth it. It wouldn't tell us anything we don't
already know.

> The oscillation in "free" and "buff" is very unusual. What is this
> box doing?

Oops, sorry. That trace is a few months old and I forgot I had used a
hack to have timestamps in vmstat. The large numbers that are alternating
are jiffies, the smaller numbers are the actual readings.

Roger