Is that from the moment we measure. Or from the moment the page was last touched ?

(This makes a difference of a factor 2).

What would the PLE be in the example below ?

6 total time that a page is expected to be in the cache.

3 the remaining time that the page is expected to be in the cache.

111111|

22222|

3333|3

444|44

55|555

6|6666

|77777

|888888

|9999999

| AAAAAAA

| BBBBBBB

^

^

time

Above example only 6 pages fit in a cache. The vertical bar is the time line.

Page two gets at that moment replaced by page 8.

Cache now time_left Total_time

1NO

2 YES 6 0 6

3 YES 5 1 6

4 YES 4 2 6

5 YES 3 3 6

6 YES 2 4 6

7 YES 1 5 6

8 YES 0 6 6

9 NO

A NO

B NO

So the average time (past) the 6 pages are in the cache is 3 seconds

And the average time (future) the 6 pages will remain in the cache is also 3 seconds.

The total time (total = past + future) that a page has remained in the cache is 6 seconds.

Maybe this wil clear up my question.

Sorry that I wasn't clear enough with my question.

Ben