Re: Why active list and inactive list?

Balbir Singh
2007-01-23T05:52:16Z

Rik van Riel wrote:
> Nick Piggin wrote:
>
>> The other nice thing about it was that it didn't have a hard
>> cutoff that the current reclaim_mapped toggle does -- you could
>> opt to scan the mapped list at a lower ratio than the unmapped
>> one. Of course, it also has some downsides too, and would
>> require retuning...
>
> Here's a simple idea for tuning.
>
> For each list we keep track of:
> 1) the size of the list
> 2) the rate at which we scan the list
> 3) the fraction of (non new) pages that get
> referenced
>
> That way we can determine which list has the largest
> fraction of "idle" pages sitting around and consequently
> which list should be scanned more aggressively.
>
> For each list we can calculate how frequently the pages
> in the list are being used:
>
> pressure = referenced percentage * scan rate / list size
>
> The VM can equalize the pressure by scanning the list with
> lower usage less than the other list. This way the VM can
> give the right amount of memory to each type.
>

This sounds like a good thing to start with. I think we can
then use swappiness to decide what to evict.

> Of course, each list needs to be divided into inactive and
> active like the current VM, in order to make sure that the
> pages which are used once cannot push the real working set
> of that list out of memory.
>

Yes, that makes sense.

> There is a more subtle problem when the list's working set
> is larger than the amount of memory the list has. In that
> situation the VM will be faulting pages back in just after
> they got evicted. Something like my /proc/refaults code
> can detect that and adjust the size of the undersized list
> accordingly.
>
> Of course, once we properly distinguish between the more
> frequently and less frequently accessed pages within each
> of the page sets (mapped/anonymous vs. unmapped) and have
> the pressure between the lists equalized, why do we need
> to keep them separate again?
>

:-)

--
Balbir Singh
Linux Technology Center
IBM, ISTL