Image magick/PHP is falling over with large images

Question

I have a PHP script which is used to resize images in a user's FTP folder for use on his website.

While slow to resize, the script has completed correctly with all images in the past. Recently however, the user uploaded an album of 21-Megapixel JPEG images and as I have found, the script is failing to convert the images but not giving out any PHP errors. When I consulted various logs, I've found multiple Apache processes being killed off with Out Of Memory errors.

The functional part of the PHP script is essentially a for loop that iterates through my images on the disk and calls a method that checks if a thumbnail exists and then performs the following:

$image = new Imagick();
$image->readImage($target);
$image->thumbnailImage(1000, 0);
$image->writeImage(realpath($basedir)."/".rescale."/".$filename);
$image->clear();
$image->destroy();  

The server has 512MB of RAM, with usually at least 360MB+ free.

PHP has it's memory limit set currently at 96MB, but I have set it higher before without any effect on the issue.
By my estimates, a 21-Megapixel image should occupy in the region of 80MB+ when uncompressed, and so I am puzzled as to why the RAM is disappearing so rapidly unless the Image Magick objects are not being removed from memory.

Is there some way I can optimise my script to use less memory or garbage collect more efficiently?
Do I simply not have the RAM to cope with such large images?

Cheers

Answer 1

See this answer for a more detailed explanation.

imagick uses a shared library and it's memory usage is out of reach for PHP, so tuning PHP memory and garbage collection won't help.

Try adding this prior to creating the new Imagick() object:

// pixel cache max size
IMagick::setResourceLimit(imagick::RESOURCETYPE_MEMORY, 32);
// maximum amount of memory map to allocate for the pixel cache
IMagick::setResourceLimit(imagick::RESOURCETYPE_MAP, 32);

It will cause imagick to swap to disk (defaults to /tmp) when it needs more than 32 MB for juggling images. It will be slower, but it will not run out of RAM (unless /tmp is on ramdisk, in that case you need to change where imagick writes its temp files).

Answer 2

MattBianco is nearly correct, only change is that the memory limits are in bytes so would be 33554432 for 32MB:

// pixel cache max size
IMagick::setResourceLimit(imagick::RESOURCETYPE_MEMORY, 33554432);
// maximum amount of memory map to allocate for the pixel cache
IMagick::setResourceLimit(imagick::RESOURCETYPE_MAP, 33554432);