pageout question

[Andriy Gapon]

on 25/07/2010 23:28 RW said the following:
> On Sun, 25 Jul 2010 17:19:41 +0300
> Andriy Gapon <avg at freebsd.org> wrote:
> 
>> on 25/07/2010 16:41 RW said the following:
> 
>>> In FreeBSD the inactive queue contains disk cache pages which
>>> normally provide most of the clean pages needed. In addition pages
>>> are dribbled out to swap, and the resulting clean pages are placed
>>> at the back of the inactive queue to make another pass. 
>> Well, "normally" and "most" are not quite quantitative.
>> Personally, I do not see any guarantees that inactive queue would
>> contain enough clean pages to reach paging target on a single pass.
> 
> I didn't say it say it was guaranteed. I just think the scenario where
> a first pass ends up between the watermarks is rare. And when it
> happens I don't see a compelling reason to do extra paging to reach an
> arbitrary target.

Well, it seems neither I nor you have data to show whether it's rare or not (and
it would greatly depend on workload too).
As to "arbitrary target" - well, that's the whole point of hysteresis-like
behavior.  We start paging also at an "arbitrary" point.

-- 
Andriy Gapon