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Note that its pretty much a must if you plan to have a MongoDB server as part of your dev environment on your home box - Mongo by design eats all the RAM it can find to map files to, and if you have Apache, Solr and MySQL running on the same box, things can get ugly :)

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Why do applications think they can manage my memory better than my kernel?

Because your kernel is general-purpose and has been tuned with certain balance between latency, throughput, and memory usage (memory management takes memory!) If your application deals with things < 4kb in size, has different performance needs &c, then it may achieve higher performance than just leaving everything up to the kernel.

Actually, MongoDB does leave memory management to the kernel by using MMAP to access data files. Most kernels will allocate a lot of memory to the disk cache, which can make it look like mongo is eating all of your ram.

This. It's one of the brilliant things about mongo. The downside is that after a reboot it's up to you to get the OS to warm those caches back up.

If all you want to do is limit memory usage, process control groups can do that with less overhead than a VM.

kernel/Documentation/cgroups/memory.txt circa line 255

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