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From Chase Venters <>
Subject Questions about optimizing memory usage
Date Thu, 15 Dec 2005 07:15:39 GMT
	I have some questions about optimizing memory usage. I could probably get 
some of these answers myself with more study of the mod_perl / perl source, 
but I was hoping someone might just know the answer and be able to tell me :)
	First off, am I correct in the assumption that it has been wise even in 
mod_perl 1 (under Apache's child-per-request model) to preload all of your 
modules for memory savings?
	Where exactly do these savings come from if the processes are forked? Is 
there some sort of mmap / shmem way that the Apache children share their Perl 
trees? Or perhaps the processes each have all that memory *allocated* 
individually, but because of COW pages from the OS, you only need one copy 
resident (hence less paging)?
	In answering the above questions - are these reasons / behaviors consistent 
with mod_perl 2 under prefork?
	Also - as of the current Perl 5.8 series, we're still not sharing / doing COW 
with variable memory (SV/HV/AV) right?
	Now as for an optimization question... if the ops in the code tree are 
shared, let's suppose I declare this subroutine via a scalar passed to eval 
prior to the clone process:

sub _get_Big_Data_Structure {
	return {
		key => {
			nested => ['values'],
			lots => {'of' => 'data'},

	The thing is that I have a big nested config structure, along with lots of 
other big nested structures. A given request doesn't need *all* of the data, 
so I've been brainstorming and thought about writing a "reduce" method that I 
would dispatch around my module tree from one of the prior-to-clone handlers 
that would take these structures, use Data::Dumper to get their "source code" 
form, and eval them into subroutines in some "stash" package. 
	I don't particularly care if this adds seconds to the Apache startup time if 
it can reduce my runtime memory usage reasonably. I suppose the only other 
thing I have to be concerned about if the above idea will work is how long 
the "_get_Big_Data_Structure" call would take.

Chase Venters

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