NBUF and pstat

[William Roberts;]

In <48252 at apple.Apple.COM> ksand at Apple.COM (Kent Sandvik) writes:

>In article <18804.2796de90 at windy.dsir.govt.nz> sramtrc at albert.dsir.govt.nz 
writes:
>This is true until you get to the point in the envelope where the 
>amount of buffers residing in memory makes it hard to find more space,
>so the system start paging, and ultimately swapping.
>And swapping should be avoided, because swapping of large binaries takes
>a long time.

Under BSD-influenced systems (including A/UX) binaries do NOT swap: the text 
segments are shareable and read-only, so the system just pulls them back in 
from the original binary. This is the reason for the "Text file busy" message; 
it would be embarassing if you changed the file from which executable pages 
get pulled back. For example:

  % cp /bin/dd /tmp/dd
  % /tmp/dd bs=10k &
  % date >/tmp/dd
  /tmp/dd: Text file busy
  %

Other things which no longer swap include the u area: this is permanently 
resident in memory under A/UX (so I'm told).

>I put together an HyperCard stack concerning A/UX tuning some time ago
>(for the A/UX sales support people in Australia). If there's interest
>I could revitalize some of the information and republish it. 

Yes please - this kind of stuff is always interesting.

>UNIX systems tuning is a black art, and with the advent of systems
>such as SysV.4 with dynamically allocated resource tables we should
>maybe get rid of that nuisance.

It would be better to get the SunOS 4.x virtual memory system, which gets rid 
of the statically allocated disk cache entirely. What effectively happens is 
that VM and Disk cache contend for the whole of the available memory, and you 
really do get compiler intermediate files never touching the disk, but without 
the overhead of a large proportion of your physical memory stolen permanently 
for disk. They make even more use of this with a trick called the "tmp 
filesystem" which resides solely in virtual memory and avoids even 
inode/directory updates going to disk: perfect for compiler intermediate files 
and other junk you don't want to keep.

As far as I understand things, this isn't part of SysV.4: but rather one of 
the "transparent extras on top of the standard" which is what product 
differentiation is all about these days.
--

William Roberts                 ARPA: liam at cs.qmw.ac.uk
Queen Mary & Westfield College  UUCP: liam at qmw-cs.UUCP
Mile End Road                   AppleLink: UK0087
LONDON, E1 4NS, UK              Tel:  071-975 5250 (Fax: 081-980 6533)