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From: gwyn@smoke.brl.mil (Doug Gwyn)
Newsgroups: comp.unix.wizards
Subject: Re: BSD virtual memory mngmt algorithm. Was: Performance Tuning Ultrix 4.1
Keywords: paging swapping fast large load BSD
Message-ID: <16121@smoke.brl.mil>
Date: 11 May 91 02:57:23 GMT
References: <1991Apr30.160331.16215@milton.u.washington.edu> <1991May7.065338.1027@ubeaut.enet.dec.com>
Organization: U.S. Army Ballistic Research Laboratory, APG, MD.
Lines: 31

In article <1991May7.065338.1027@ubeaut.enet.dec.com> chris@ubeaut.enet.dec.com (Chris Jankowski) writes:
>1. Did other vendors (who derived their commercial U*X offerings from 
>BSD) modify or tune the virtual memory subsystem to significant
>extent perhaps providing new nifty algorithms?

Other than Sun, almost all ports of 4.nBSD have had to overhaul the
virtual memory code, which was written quite narrowly for a VAX-like
VM system.  (Sun simply BUILT a VAX-like VM system!)

>2. What does it looks like for ATT System V rel. 3 and 4?
>Do they have very different virtual memory management?

Yes, and furthermore SVR3 and SVR4 differ from each other.
SVR3 used a region-based scheme (similar to VAX/VMS), while SVR4
uses an adaptation of SunOS's VM system.  The region scheme is,
I believe, portable to a wider range of architectures.

>3. What is the state of the art in this area considering vastly
>increased demands on the virtual memory subsystem imposed by
>X.11, RDBMS, transaction processing and the like applications?

Shared libraries and dynamic linking have had a bigger effect
than silly applications.  In general, if the sum of all task sizes
exceeds the amount of physical memory by very much (say a factor
of two), the system is likely to bog down no matter what VM
algorithms are employed.

>4. Do other operating systems using virtual memory approach differ
>much in the way of the algorithms used for virtual memory management?

Yes.