VM balancing problems under 2.4.2-ac1

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and elsewhere,

Rik, can you explain why we still need to "balance" things, instead of just swapping out the least used pages? Is this only a problem with the 2.4 implementation (e.g. will refill_inactive_scan eventually do swap_out in 2.5, or something?), or is it a generic VM issue that has to be solved?

I'm not quite sure what is going on, but is there perhaps some way that we can get better information about recent use for pages? Or do we have to treat age pages of different type differently based on their usage pattern, which depends on the type of workload?

Regarding better information on recent page accesses, one thing that seems strange to me is aging pages down to 0 before making them inactive. In order to have the most amount of information in the page ages, we don't want more than 1/PAGE_AGE_MAX pages to have the same age. (Well, there are problems to that view, ) If we only consider pages with age 0 to be inactive, then if we want a lot of inactive pages, we make all those pages age zero. Instead, it seems like a good thing to do, would be to have a variable "inactive_level", so that a page is inactive if the age is less than inactive_level. If we want more pages to be inactive, we increase inactive_level. I guess I can thing of some problems for this approach, but maybe it can be rescued.

In any case, is there a file in /proc/ that displays the number of pages at each age? It could be interesting to calculate Shannon Entropy from this and see how many bits of information we get - a better entropy could indicate a better aging algorithm. I guess "6" is the best we could get right now.

Also, somebody posted a paper reference to linux-mm a few months ago, about how the optimal page aging strategy aged pages according to how rapidly referenced bits were being set. I guess the background page aging tries to do something like this. I don't know how well it succeeds though...

Thanks for any response/explanation!

-BenRI

"...assisted of course by pride, for we teach them to describe the
 Creeping Death, as Good Sense, or Maturity, or Experience." 
- "The Screwtape Letters"
Benjamin Redelings I      <><     http://www.bol.ucla.edu/~bredelin/
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The problem is that we scan processes by _virtual address_ and the cache by physical page. Furthermore, there are LOTS of pages which are present in both the cache AND in processes.

Say, for example, that on a 64MB system you have 32MB cache but half of that cache is cached executable text and swap cache (which is also mapped into processes).

Now how would you adjust the scanning rate of processes and cache to make sure we age each page in the system at approximately the same rate ?

regards,

Rik
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