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impressions on gc

From: Andy Wingo
Subject: impressions on gc
Date: Fri, 02 Dec 2011 00:09:12 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.3 (gnu/linux)


I recently poked some GC-related things in stable-2.0 and master.

One change was to make GC run more often when a process is growing, in
terms of resident memory size.  This seems to be a good idea in general.

The other was to add a function that users can call to note
non-gc-managed allocations -- allocations that may be freed when GC is
run, but which the GC doesn't know about.  That is
scm_gc_register_allocation.  This routine runs GC after allocated bytes
exceed some limit, currently the GC-managed heap size, reset after every
GC.  This is to catch steady-state mallocation.

We cannot deal well with steady-state mallocation, unknown to the GC.
If the ratio between unknown and known allocation is small enough, the
process image size won't grow too badly.  But if it is large, your
process will grow quite large.  You have to let the GC know about
allocation -- at least, about allocation that can be freed by GC.
Otherwise it can't do its job.

Those are my conclusions.  It is midnight.  Goodnight! ;-)


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