http://www.perlmonks.org?node_id=707207

On 1 of the newsgroups, someone asked about the pros/cons differences between using forks or threads. Well the newsgroup is c oriented, and I thought I would mention the way that interpreted languages (like Perl) will hold onto memory released by threads. Specifically I mentioned that if a high memory usage thread is run first, then a series of low memory usage threads, the memory of the process will stay at the high memory usage. I used the following script to demonstrate. (I have a 1 gig ram on a linux 2.6.22 kernel). The results were unpredictable, but generally followed the rule that heavy mem use is freed, medium use is half freed, and light use ( <5%) is not freed( but may be on occaision).
#!/usr/bin/perl use warnings; use strict; use threads; my $count = 0; # load a mem intensive thread first, then watch how mem drops # notice the difference in memory reclamation( top) after # the following thread is run with heavy,medium, light mem # on my machine # heavy mem use will skyrocket to 75%, then drop to 2% # medium will jump to 40% then drop to 20% # light use will jump to 5% then stay there # none use will stay at .3 % #my $t = threads->new(\&do_my_thing, $count, 1000000); #light #my $t = threads->new(\&do_my_thing, $count, 4000000); #medium my $t = threads->new(\&do_my_thing, $count, 8000000); #heavy #my $t = threads->new(\&do_my_thing, $count, 1); #none $t->join; select(undef,undef,undef,.1); while (1) { $count++; print "thread $count started\n"; my $t = threads->new(\&do_my_thing, $count, 1 ); $t->join; select(undef,undef,undef,.1); } <>; #wait for keypress to end, to check top sub do_my_thing { my ($val,$cdown) = @_; my %hash; for(0..$cdown){ $hash{$_} = 'a'; } select(undef,undef,undef,.1); print "thread $val ended\n"; }
So it seems that the kernel is deciding how much to free, depending on some internal formula.

The question is: Can we trust top/ps to really report what is going on. Since most threaded scripts may go 10% up occaisionally, if they don't reach the kernel threshold for freeing, is that dead memory, or will the kernel auto-free more (the diff between 5% and .3%) if it needs the memory for other apps.

This would be great news for threaded apps, but I worry about not being able to trust top/ps, or the kernel doing things behind the scenes.

Anyways, this may be a mechanism to reclaim memory in Perl. I would appreciate any thoughts, comments on what is happening.


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