Swapping out old pages

Swapping out old pages

[Karl Vogel]

I recently became interested in all this VM stuff, so I'm not what you would
call an expert (more like a newbie), but when browsing through the 2.2 mm
sources, the following questions popped into my mind:

- why does the swap out algorithm select the task with the largest RSS to
free pages. If I'm not mistaken, the age of a page isn't considered?! Why is
that? Am I overlooking something? (doesn't this mean that if I start a new
large process, it's pages immediately get swapped out even though there are
other processes that haven't done anything in the past couple of hours)

- wouldn't it be beneficial if there is a parameter that allows you to
specify that after a certain age, a page is swapped out to make room for the
buffer cache. (even if the system has plenty of ram left - the idea is that:
if your system has alot of RAM, old/unused pages (e.g. init code from
daemons etc) are never swapped out and take away ram that can be used for
better things).




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Re: Swapping out old pages

[Rik van Riel]

On Tue, 18 May 1999, Karl Vogel wrote:

> - why does the swap out algorithm select the task with the largest
> RSS to free pages. If I'm not mistaken, the age of a page isn't
> considered?! Why is that? Am I overlooking something?

You're overlooking the fact that p->swap_cnt is only
recalculated when all processes have been scanned.

A new sweep starts at the process with the largest
swap_cnt -- but all processes are scanned eventually.

> - wouldn't it be beneficial if there is a parameter that allows
> you to specify that after a certain age, a page is swapped out to
> make room for the buffer cache.

The system already does that, and has been doing so
rather agressively since 2.1.89...

cheers,

Rik -- Open Source: you deserve to be in control of your data.
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