I’ve been looking for this over and over and over, until I had some inspiration today (thanks to Andew and Chris) .. this has one and only one sole purpose: safekeeping, so I don’t end up searching for it all over again …

To free pagecache:

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# sync; echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes:

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# sync; echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes:

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# sync; echo 3 > /proc/sys/vm/drop_caches

Once that is done, you’ll see the memory (usually RAM) freeing up.