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From: dolson@ADA20.ISI.EDU.UUCP
Newsgroups: mod.computers.vax
Subject: Re: Sizes of look-aside lists
Message-ID: <8701310341.AA18798@ucbvax.Berkeley.EDU>
Date: Fri, 30-Jan-87 16:01:03 EST
Article-I.D.: ucbvax.8701310341.AA18798
Posted: Fri Jan 30 16:01:03 1987
Date-Received: Sat, 31-Jan-87 09:18:58 EST
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>From: JCV@CERNVM.BITNET
>Subject: Re: Sizes of look-aside lists
>
>I disagree with Kevin Oberman's and Mark Johnson's observations
>on the size of the nonpaged pool look-aside lists. I can see no
>reason why they should be any larger than the number you really
>require and a number of reasons to make them as small as possible
>(even making the ...V parameters as small as possible).
>
  [...] 
>
>3. If expansion is necessary, VMS will steal a page from the free list,
>   make the appropriate entry in the system page table valid and
>   format the new memory as necessary. This will waste the previously
>   allocated PFN database for that page, which is 18 bytes (22 bytes
>   on machines with more than 32 MB of memory). This is only for those
>   pages added when pool is expanded, as the space allocated during
>   booting is never accounted for in the PFN database.
>   Note that the actual operation of increasing the list is only done
>   once and is quite fast (no I/O, for instance).

Well...there is room for disagreement here.  "If expansion is necessary"
then your machine is presumably loaded, all its request packets of one
sort or another are in use, and its the worst possible time for VMS to
steal a page from the free list; its already behind on doing the work
its supposed to be at, or it wouldn't have all those request packets
waiting around holding temporary data.  I would NOT agree that this is
"quite fast", because you are imposing additional system overhead at
exactly the worst time, when your cpu cycles are in shortest suppply.
If it HAS to happen, I'd rather arrange pool space at startup, than
choose to rearrange it when it runs out...

Doug Olson
Project Manager
AFCMD, Kirtland AFB, NM
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