Messages in this thread | | | Date | Sun, 28 Dec 2003 18:15:12 +0100 | From | Roger Luethi <> | Subject | Re: Page aging broken in 2.6 |
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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.
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