PHP ImageMagick use 10gb~ memory when resizing a big image - resize

Question

I have problem when I try to resize a very big image like a 30,000px x 20,000px with ImageMagick software. But I do not have enought RAM. I have 8GB, but the software requires about 10GB~. I need to use all data in RAM. I tried two options:
1 using convert, writes to a different image file:
convert -resize 90% source.jpg destination.jpg
using mogrify overwrites the original image file
mogrify -resize 90% source.jpg
Maybe someone knows how I can use less memory or some other tricks.
More information about Imagemagick: http://www.imagemagick.org/script/mogrify.php

Answer 1

If your attempting to configure ImageMagick's memory polices to use more RAM, before caching to disk, define the AREA limit by modifying policy.xml or set an environment variable.
Setting Memory limit in Environment
This is probably the quickest way for one-off process.
MAGICK_AREA_LIMIT=8GB convert -resize 90% source.jpg destination.jpg
Also attempt adjusting other environment vars
MAGICK_MAP_LIMIT - Maximum amount of memory map in megabytes to allocate for the pixel cache.
MAGICK_MEMORY_LIMIT - Maximum amount of memory in megabytes to allocate for the pixel cache from the heap.
Setting Memory limit with Policy.xml
Edit the policy.xml under your {$PREFIX}/lib/ImageMagic-X.X.X/config directory.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE policymap [
<!ELEMENT policymap (policy)+>
<!ELEMENT policy (#PCDATA)>
<!ATTLIST policy domain (delegate|coder|filter|path|resource) #IMPLIED>
<!ATTLIST policy name CDATA #IMPLIED>
<!ATTLIST policy rights CDATA #IMPLIED>
<!ATTLIST policy pattern CDATA #IMPLIED>
<!ATTLIST policy value CDATA #IMPLIED>
]>
<policymap>
<policy domain="resource" name="memory" value="8GB"/>
<policy domain="resource" name="map" value="8GB"/>
<policy domain="resource" name="area" value="8GB"/>
</policymap>
Verification
Whatever method you choose, you can verify by running the identify utility.
$ identify -list resource
File Area Memory Map Disk Thread Throttle Time
--------------------------------------------------------------------------------
1920 8GB 8GB 8GB unlimited 1 0 unlimited

Answer 2

It's not clear what you are trying to achieve overall, but you can resize an image while reading it like this and it should take less memory:
convert source.jpg'[15000x10000]' destination.jpg

Answer 3

Edit the policy.xml which resides under /etc/ImageMagick/policy.xml
In my case it was using more than 2GiB, but here you can limit so that script should be running under the provided limit. I set it for 512MiB for map and memory.