You used the same image with the same libraries, it means libgd and libpng have no restrictions about the memory size (or at least not one for the size you're using), the problem may be php but you are sure the memory limit is 650M (if your php installation is using suhosin, you may want to check suhosin.memory_limit)
You tell the kernel is killing the script (but I can't find why you are telling so, is there a dmesg log about?) but the kernel kill a process only when is out of memory.
One problem could be memory fragmentation / contiguous alloc, nowadays it should be more a kernel space problem than user space, but the size your script is allocating is not an every day size.
Fragmentation could be a problem if your script is running on a server with a long uptime with a lot of process allocating and freeing memory, if is a virtual machine that you just start up and the script fail at the first launch, then the problem isn't fragmentation.
You can do a simple test, command and code below, it use the gcc compiler to create (in the directory where you launch the command) a a.out executable which allocate and set to 0 a memory chunk with size 191 x 1024 x 1024 (about ~190M)
echo '#include <stdlib.h>\n#include <string.h>\n#define SIZ 191*1024*1024\nint main() {void *p=malloc(SIZ);memset(p,0,SIZ);return (p==NULL?1:0);}' \
| gcc -O2 -xc -
run the executable with:
./a.out
If the problem is the system memory the kernel should kill the executable (I'm not sure 100%, the kernel have some policy about which process to kill when there's no memory, to be checked)
You should see the kill message though, with dmesg perhaps.
(I think this shouldn't happen, instead the malloc should just fail)
If the malloc succeed, the executable can allocate enough memory, it should return 0, if it fail should return 1
to verify the return code just use
./a.out
echo $?
(don't execute any other command in the middle)
If the malloc fail you probably just need to add physical or virtual memory to the system.
Keep in mind that ~190M is just the memory for one uncompressed image, depending on how php, libgd and libpng are working the memory for the whole process may be even twice (or more).
I did a simple test and profiled the memory usage, the peak with an image 7600x2200 (1.5M on disk) on my system is about ~342M, here's the test:
<?php
$im = imagecreatefrompng("input.png");
$string = "Sinker sucker socks pants";
$orange = imagecolorallocate($im, 220, 210, 60);
$px = (imagesx($im) - 7.5 * strlen($string)) / 2;
imagestring($im, 3, $px, 9, $string, $orange);
imagepng($im);
imagedestroy($im);
I tried xhprof at first but it returned values too low.
So I tried a simple script memusg
memusg /usr/bin/php -f test.php >output.png
(the test work btw)
