PK> There was a fundamental change in the logic of the USE of SWAPPER.DAT
 PK> somewhere between V2.0 and V2.11 but I can't remember enough of the
 PK> details (it was too long ago) of when all this changed. The main
 PK> change was that a PAGE in memory that was from the EXECUTABLE part of
 PK> a program that NEVER altered, was written to the SWAP file just ONCE,
 PK> and subsequently was never written again. This provided a small
 PK> performance improvement on systems that were doing a lot of swapping.

In other words: discardable pages, aren't.

I believe (from memory) that the change was that discardable pages were not
discarded when paged out, but instead were written to the swap file. 
Discardable pages were, prior to that, re-created from the compressed page
image in the executable file afresh every time that they were demand paged in. 
 But uncompressing page images could have a significant performance impact on
systems where paging rates were high, as would usually be the case with (say)
6MeB machines, since it would be done for each page-in of a discardable page.

Changing OS/2 so that it only had to uncompress the image of such a page once, 
the first time that it was demand paged in, removed this overhead.

 ¯ JdeBP ®

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