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Is it enough to have the entire index in memory/ram or does mongodb even try to allocate as much ram as possible to store even the data for fast reads?

I'd like to run mongodb + other applications and it looks like mongodb is the only one which does not allow me to define a range of RAM, to lets say "max_memory_allocated_or_reserved=8GB".

If there is no way to do so, I should explain to oom-killer that mongod is the "bad" process which is not best practise in my opinion...

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up vote 6 down vote accepted

The real reason why you can't do as you ask (limit the memory) is because MongoDB doesn't manage the memory it uses directly - it lets the OS do it. MongoDB just memory maps all its data and then has the OS page it in and out of memory as needed. As a result, there is no direct management of the amount used possible until MongoDB implements this in a completely different way, or the OS allows it (not possible in Linux since the 2.4 days).

The only way to truly segregate resources at present is to use a virtualization solution and isolate MongoDB in its own VM. Yes, there are overheads involved (though hypervisors have gotten a lot better), but at the moment that is the price to be paid for that level of resource control.

In terms of the OOM Killer, even with no other processes on the host, as long as your data set and indexes overall exceed available memory, MongoDB can hit OOM Killer issues. This is because of how the data gets paged out of memory - if there is no memory pressure (nothing else wants Resident memory), and you keep adding/touching new data and indexes, then eventually it will grow to consume all available RAM. Hence the recommendation to always configure some swap when running MongoDB:

http://www.mongodb.org/display/DOCS/Production+Notes#ProductionNotes-Swap

Of course, LRU data will be paged out first, other processes can take up the res mem also, but the concept still applies unless you load your data set into memory and then it stays static. Best thing to do if you are worried is get it into MMS and track the usage over time:

http://mms.10gen.com

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MongoDB will use available free memory for caching, and swap to disk as needed to yield memory to other applications on the same server. For the best performance you'll want to have enough RAM to keep your indices and frequently used data ("working set") in memory.

Helpful reading:

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