Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!hoptoad!tim
From: tim@hoptoad.uucp (Tim Maroney)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Handles and Virtual Memory
Message-ID: <9415@hoptoad.uucp>
Date: 26 Dec 89 23:14:41 GMT
References: <3875@atr-la.atr.co.jp> <4525@helios.ee.lbl.gov>
Reply-To: tim@hoptoad.UUCP (Tim Maroney)
Organization: Eclectic Software, San Francisco
Lines: 38

In article <4525@helios.ee.lbl.gov> beard@ux1.lbl.gov (Patrick C Beard) writes:
>With virtual memory on the horizon, Handles may not be long for this world.

Generally a good message, but I wanted to take issue with this.  In the
new Mac virtual memory system, applications can't increase their heap
allocations at runtime.  Therefore, heap fragmentation can still be a
problem, and handles remain important.

It is a mistake to extrapolate from other VM systems to the Mac's,
which is rather limited.  For instance, under ordinary VM, a RAM-based
editor is automatically a disk-based editor.  Many large documents can
be kept in RAM, apparently, even though in fact they are being kept
largely in paging space on the disk.  With the Mac VM system, though,
you are limited to your initial allocation of RAM.  So a RAM-based
editor does not approximate to the performance of a disk-based editor,
and you would still be well advised to handle your own temporary files,
purgeable lines, and so forth.

I am also a bit concerned about the performance of the Memory Manager
under VM.  The Mac uses a linked-list block allocation scheme, which
involves searching through linked lists of free blocks in RAM for
allocation operations.  This is suboptimal for a VM system -- there may
be a lot of paging overhead as you go through the linked list, and your
page reference counts will be affected strangely.  For VM, it is
generally better to use an index-based allocator, where the information
on free blocks is kept separate from the blocks themselves and
concentrated in a few pages of indices.  This way, free blocks do
not have to be paged in to be checked for suitability.

Does anyone have any information on the performance impact of the Mac
MM algorithm under VM?
-- 
Tim Maroney, Mac Software Consultant, sun!hoptoad!tim, tim@toad.com

"Because there is something in you that I respect, and that makes me desire
 to have you for my enemy."
"On those terms, sir, I will accept your enmity or any man's."
    - Shaw, "The Devil's Disciple"