Why the real memory usage of ImageMagick is higher than what I expected?

Question

https://stackoverflow.com/a/26849250

According to the answer above, a Q16 version of ImageMagick needs 8 bytes of memory space per pixel. Now I use Q16 version of ImageMagick, and I have an image (XXX.jpg), which size is 5680 x 3787 (px). Below is my PHP code of ImageMagick:

$file = "XXX.jpg";
$image = new Imagick();
$image->readImage($file);

$type = $image->getImageFormat();
$image->setFormat($type);

echo $image->getImagesBlob();

Theoretically, it should take 5680 * 3787 * 8 = 172081280 bytes = about 164.11 MB of memory usage. But I look up the memory usage statistics, the virtual memory usage takes up to 315.03 MB and the physical memory usage takes up to 349.71 MB. The usage amount is extremely higher than what I expected. What happened to the memory usage? Is there any miscalculation?

By the way, how to calculate the CPU usage of ImageMagick?

P.S. Below is the PHP information of my ImageMagick:

Not only the image consumes memory, the code uses memory too. — Marged, Jul 29 '17 at 15:19
Is it possible to calculate the total memory usage including the image and the code? — blue cat, Jul 29 '17 at 15:24
You're loading and decompressing a jpeg file too. That needs memory as well. — Álvaro González, Jul 29 '17 at 15:27
Q16 ImageMagick needs 16 bytes per pixel ***per channel*** for the input (after decompression) and also similar for the output and for any temporary images created by clones in the command line. — fmw42, Jul 29 '17 at 17:41
Glenn, thanks for the correction. That was what I wanted to say, but it came out wrong. — fmw42, Jul 30 '17 at 00:52
Note that you can get and set resource limits (in terms of RAM, threads, etc) in your code `Imagick::getResourceLimit(Imagick::RESOURCETYPE_MEMORY);`, or in your config files - though that may slow things down if you force it to go to disk. — Mark Setchell, Jul 30 '17 at 10:36

Why the real memory usage of ImageMagick is higher than what I expected?

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