Limiting ImageMagick Memory Use

Question

Basically I'm trying to keep the memory use on my Nginx server under a certain amount, both because I'm insane (according to my friends) & I want to save money. However I'm worried ImageMagick may push it over the edge.

I'm using -limit area 20MiB and I've also tried -limit memory 15MiB -limit map 15MiB but when checking the process (as it runs) through top -c (with Shift-M) and ps aux it shows it using, sometimes, considerably more memory than I've set in the limits. To give numbers it may be using 35MB or 40MB, instead of the 20MB/30MB I would expect. I wouldn't be bothered for 2MB or 3MB but that's quite a large offset.

I've been told the extra memory may be the ImageMagick's overhead as it loads the interpreter etc, but I'm not super familiar with Unix programs so haven't a clue in that department.

If anyone can explain why this is happening, that would be great. If it's a normal thing, great. I'll just adjust things to take into account the fact that it may use my limit plus a certain amount, but if it isn't and the -limit parameter doesn't limit memory to a certain amount, what exactly is the point in having that parameter in ImageMagick?

Again thanks for your help in advance, it's much appreciated, as always.

Answer 1

According the documentation ImageMagick is moving all memory operations to mmaped files, so it will start to swap if you have enough disk space, see the manual: SNIP from manual -limit:

The value for File is in number of files. The Disk limit is in Gigabutes and the values for the other resources are in Megabytes. By default the limits are 768 files, 1024MB memory, 4096MB map, and unlimited disk, but these are adjusted at startup time on platforms that can provide information about available resources. When the limit is reached, ImageMagick will fail in some fashion, or take compensating actions if possible. For example, -limit memory 32 -limit map 64 limits memory When the pixel cache reaches the memory limit it uses memory mapping. When that limit is reached it goes to disk. If disk has a hard limit, the program will fail.