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Date:Mon Oct 19 09:24:33 1993 
Subject:Garbage collection 
From:Robin Popplestone 
Volume-ID:931019.01 

Helen McCall writes:

> My experience is  that the  performance penalty is  not all  one suffers
> with automatic garbage collection in POPLOG.

See Steve Knight's contributions on whether there is penalty or gain...

>  POPLOG may well collect up garbage  and free the memory for it's  own
> use, but it  does not  appear to be  very altruistic  with respect  to
> other  processes  running  on  the  machine.  When  making  a  garbage
> collection, POPLOG actually INCREASES the total amount of memory it is
> using, so forcing other processes running on the machine into an  "out
> of swap space" situation and crashing them.

I have  never  experienced this  on  any machine  I  have used  Poplog  on,
including my laptop... Clearly it is a matter of system configuration,  and
I have  always used  Unix systems  configured for  people with  elephantine
processes (many lisps are far worse). It  is true of course that Poplog  is
expensive to have around in swap-space as in other things. The  non-copying
collector is probably quite a good idea for many purposes.

> Further to  this; I  have found  that POPLOG's  own behaviour  becomes
> somewhat unpredictable and irrational when swap space runs low, with a
> distinct tendency for conditional branches to default to the else path
> regardless of  the evaluation  performed, and  without signalling  any
> error.

This has to be a case of blaming the victim. You don't say which operating
system is involved, but it -must- be the fault of the operating system
if a user process exhibits any behaviour like this in response to a
condition like low swap space.

Certainly I have experienced GC trouble with POPLOG running under A/UX,
which was at the time an apology for a Unix (I don't know what it is like
now). The trouble did clear up when I turned off the copying collector.
But it was quite clear that A/UX was the villain.

> Allowing  the  programmer  to  manage  the  utilisation  of  resources
> herself, can save an awful lot of heartache!

Well indeed - why don't you throw away the operating system too, and keep
hands on control of all those nice glossy devices?


Robin