Path: utzoo!utgpu!watserv1!watdragon!watsol!tbray
From: tbray@watsol.waterloo.edu (Tim Bray)
Newsgroups: comp.unix.wizards
Subject: Gripe about mickey-mouse VM behaviour on many Unixes
Keywords: MIPS, System-V, paging performance, working-set
Message-ID: <19821@watdragon.waterloo.edu>
Date: 16 Jan 90 03:28:19 GMT
References: <1990Jan15.184042.7564@keysec.kse.com>
Sender: daemon@watdragon.waterloo.edu
Reply-To: tbray@watsol.waterloo.edu (Tim Bray)
Distribution: world
Organization: U. of Waterloo, Ontario
Lines: 38

gm@keysec.kse.com (Greg McGary) writes:
>Our MIPS M/120 has been spending too much time page-thrashing lately.
>I would like to have a utility that tells me about the paging behavior...

Good luck, you'll need it.  Here at the New OED project we have got seriously
cheesed off about VM implementations on many unix systems.  No matter how much
you have, memory remains a critical resource.

But even well-regarded Unixes don't give you the tools to manage it.  In many
applications (e.g. database indexing) performance of algorithms can be greatly
improved by knowing how much physical memory you can use, and tuning to use it 
efficiently.  But not on Unix.  Some case studies:

1. On a 32-Mb (4.3bsd) machine with *nothing* else happening, the OS stupidly 
   pages away at you if you try to use more than about 20 Mb in the inane 
   belief that the memory will be needed any moment for one of those gettys or
   nfsds or something that aren't doing anything.

2. A process using only a moderate amount of memory (you think) runs like
   a dog, and you note that the system is spending much of its time in
   system state or idle.  Why, you wonder.  It quickly becomes apparent 
   that the information produced by items such as ps, vmstat, vsar, top, 
   and so on, is comparable in relevance and accuracy to Robert Ludlum novels 
   or peyote visions.  (SunOS the villain here).

3. On a 64-Mb (MIPS) machine, your paging rate, system time, and idle time 
   all go through the roof if your process insolently tries to random-access
   more than 32 Mb of memory at once.

Look, we all appreciate the tender loving care that VM architects have put
into strategies that are friendly to 100+ moderate-size processes context
switching rapidly in time-sharing mode.  But there are other ways to use
computers, and they are currently very poorly supported.  We paid for that
memory, we have a good use for it, and the OS is getting in our way, and it's
also REFUSING TO TELL US ACCURATELY WHAT'S GOING ON - an unforgiveable sin by
my Unix dogma.

Harrumph, Tim Bray, New OED Project, U of Waterloo