I'm doing some research in perl's (and by extension mod_perl's) use of
memory. I recently used the terrific B::TerseSize module to profile a
number of modules used in my server and found some discrepency in
sizes. The total bytes used was greater than the sum of the
individual elements.
Digging deeper I found that the module in question initialized a
package variable when the module was loaded. I moved the code into a
BEGIN block and the memory usage dropped.
I then performed a test case (see below) and verified that this is the
case.
I've not seen this mentioned as of yet, but I think it's a useful
technique to reduce the memory constraints of your mod_perl based
server.
Of course, this begs the question, why doesn't Perl free up the memory
of code placed inside modules?
Anyway, here's the proof, done on a RedHat 7.1 system with perl 5.6.0.
The module TestMem (attached) was run with and without inititialization
code in a BEGIN block. To test I ran
perl -MTestMem -e 'TestMem::foo'
I then compared the output of the /proc/PID/statm
The results, without and with a BEGIN block are:
305 305 261 117 0 188 44
296 296 251 118 0 178 45
The fields here are:
size total program size
resident size of in memory portions
shared number of the pages that are shared
trs number of pages that are 'code'
drs number of pages of data/stack
lrs number of pages of library
dt number of dirty pages
So, we save about 9k just by moving code into a BEGIN block for this
simple example..
----------------------------------------------------------------------
package TestMem;
my $a = 1;
my $b = 10;
#BEGIN {
$a = $a + 10;
$a = $a * 10; $a = $a / 9; $a = $a * 10; $a = $a / 9;
$a = $a * 10; $a = $a / 9; $a = $a * 10; $a = $a / 9;
$a = $a * 10; $a = $a / 9; $a = $a * 10; $a = $a / 9;
$b = $b * 100; $b = $b / 100; $b = $b * 100; $b = $b / 100;
$b = $b * 100; $b = $b / 100; $b = $b * 100; $b = $b / 100;
$b = $b * 100; $b = $b / 100; $b = $b * 100; $b = $b / 100;
$b = $b * 100; $b = $b / 100; $b = $b * 100; $b = $b / 100;
$b = $b * 100; $b = $b / 100; $b = $b * 100; $b = $b / 100;
$b = $b * 100; $b = $b / 100; $b = $b * 100; $b = $b / 100;
# } # end begin..
sub foo {
my $c = " " x 10000; # hopefully reallocate freed mem...
print "pid $$ : $a $b";
sleep(10000000);
}
1;
--
Paul Lindner
[EMAIL PROTECTED]
