> When memory becomes low, a kernel process "pagedaemon" (I think) is
> woken up. It first scans the list of "to-be-free-list", moving 'x'
> pages from the head of the list into the kernel's free-list. By
> taking from the head of the list, pages which have recently been added
> are given a chance to be reclaimed by the process(es). If free memory
> is becoming very low, then the dirty pages are written to disk and
> moved to the end of the "to-be-free-list".
>
> The "clock" is a "Not-Recently-Used" algorithm, and is not very
> difficult to implement. It can run of the kernel's soft-interrupt.
> The main difficultly is in "tuning";
> How many pages to scan each time (ie. how far to move the
> hands).
>
> markhe
In my VAX/VMS days, there were two type of page-faults:
"hard", where the page had to be read in from disk, and
"soft", where the page was still in memory (and on disk), but on the list of
pages that had been written out but not yet re-used.
As I recall, a soft-page-fault was so cheap, that they didn't worry too much
about it.
Ray