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From: Krzysztof Duleba <krzysan AT skrzynka DOT pl>
Subject:  Re: perl - segfault on "free unused scalar"
Date:  Thu, 28 Jul 2005 00:14:02 +0200
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Igor Pechtchanski wrote:

>>>>>$ ./inter.pl
>>>>>perl> sub foo($){$a=shift;foo($a+1);}
> 
> You do realize you have infinite recursion here, right?

Sure.

>>>>Segmentation fault (core dumped)
> And this is Windows saying "I don't think so". :-)

:-)

>>>I don't know.  Maybe it is a Windows feature that applications running
>>>out of memory are crashing?
>>
>>But there's plenty of memory left when perl crashes. I have 1 GB RAM and
>>1 GB swap file.
> 
> IIRC, unless you specifically increase heap_chunk_in_mb, Cygwin will only
> use 384M of address space (which seems consistent with the sbrk() and the
> request size above).

I thought of that. However:

$ cat foo.c
#include <stdlib.h>

int main(int argc, char * argv[]){
         int i;
         char * ptrs[1024];
         for(i = 0; i < atoi(argv[2]); ++i){
                 ptrs[i] = malloc(1024 * 1024 * atoi(argv[1]));
                 memset(ptrs[i], 'a', 1024 * 1024 * atoi(argv[1]));
         }

         sleep(10);
}

$ ./foo 200 5

$ ./foo 800 1

$ ./foo 2 500

I've been using more than 384 MB in C and C++ in Cygwin for a long time. 
Why heap_chunk_in_mb would affect Perl, but not C?

>>I've simplified the test case. It seems that Cygwin perl can't handle
>>too much memory. For instance:
>>
>>$ perl -e '$a="a"x(200 * 1024 * 1024); sleep 9'
>>
>>OK, this could have failed because $a might require 200 MB of continuous
>>space.
> 
> Actually, $a requires *more* than 200MB of continuous space.  Perl
> characters are 2 bytes, so you're allocating at least 400MB of space!

Right, UTF. I completely forgot about that.

> FWIW, the above doesn't fail for me, but then, I have heap_chunk_in_mb set
> to 1024. :-)

I'll try that in a while.

>>But hashes don't, do they? Then why does the following code fail?
>>
>>$ perl -e '$a="a"x(1024 * 1024);my %b; $b{$_}=$a for(1..400);sleep 9'
> 
> Wow.  You're copying a 2MB string 400 times.  No wonder this fails.  It
> would fail with larger heap sizes as well. :-)
> 
> This works with no problems and very little memory usage, FWIW:
> 
> $ perl -e '$a="a"x(1024 * 1024);my %b; $b{$_}=\$a for(1..400);sleep 9'

I didn't use references on purpose. I wanted to avoid the problem that 
arrays require continuous space, so using an array to measure system memory 
capacity is inaccurate. On the other hand, hash is a pointer structure (at 
least I think so), so it should work with fragmented memory.

I don't see why "no wonder it fails", unless it's a reference to 
aforementioned heap_chunk_in_mb.

>>Or that one?
>>
>>$ perl -e '$a="a"x(50 * 1024 * 1024);$b=$a;$c=$a;$d=$a;$e=$a;sleep 10'
> 
> Yep, let's see.  100MB * 5 = 500MB.  Since Cygwin perl by default can only
> use 384MB, the result is pretty predictable.  Perl shouldn't segfault,
> though -- that's a bug, IMO.

Should I do anything about it?

>>On linux there's no such problem - perl can use all available memory.
> 
> Yeah.  Set heap_chunk_in_mb to include all available memory, and I'm sure
> you'll find that Cygwin perl works the same too.  However, you might want
> to read some Perl documentation too, to make sure your data structure size
> calculations are correct, and that your expectations are reasonable.

Thanks for being so helpful. That really explans a lot. Thanks to Dave and 
Gerrit, too.

Krzysztof Duleba


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