Mail Archives: cygwin/1998/02/05/14:30:04
> Discarding pages in strict FIFO order seems to me like a recipe for
> thrashing.
I don't know about Win95, but VMS and (its closely related
descendant) NT make no attempt to maintain a working set in the
conventional (Denning*) meaning of the phrase:
pages are trimmed from processes when the system gets
short of memory, but not on any kind of LRU basis
these pages are sent to a kind of limbo list* with
their contents intact
if a page faults and is on the limbo list, it's
given back to the process (this is called a soft
fault and is MUCH cheaper than having to read the
page from disk)
pages that stay on the limbo list too long end up
getting reused for new pages
Obviously there's a tradeoff between the overhead of maintaining some
kind of LRU ordering and the overhead of taking lots of soft faults.
Does anybody know of any studies on this tradeoff?
If the perfect is the enemy of the good and the good is the enemy
of the good enough, the soft fault approach is probably good enough.
However, my experience with software that strongly exploits VM (e.g.,
Lisp environments) is that such software performs much better in a
conventional working set environment (say, Twenex) than it does in a
soft fault environment (say, VMS).
*see http://paris.lcs.mit.edu/~bvelez/std-colls/cacm/cacm-1751.html,
for one
*NT calls this the standby list - go to the Performance Monitor,
select Object: Memory Counter: Page Faults/sec and hit Explain.
Also, AltaVisting for "standby list" and "soft fault*" turned up:
http://www.tek.com/VND/Support/Release_Notes/PERFMON.html and
http://www.developer.com/reference/library/0672309777/mbaxc.htm
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"oncology recapitulates philately" --Mark Maxson Robert M. Praetorius
"balance, not symmetry" --Mark Stanley work: RPraetorius AT AspenRes DOT Com
(attribution by Stigler) fun & recreation: rmp AT PopJ1 DOT MA DOT UltraNet DOT Com
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