memory-safe mongodb tasks - mongodb

Question

MongoDB is running out of memory on a 96GB root server when adding a single index on a timestamp field for a 50GB collection.
Does MongoDB have any option to run a query or task in "safe-mode", e.g. without cutting the memory too much? It seems to be very touchy and can be crashed, e.g. by running some find queries with $lte/$gt on a non-indexed timestamp field.

Answer 1

i can't control it, but shouldn't there a mongodb config setting for "safety" which makes sure to release RAM once it's breaking the limit? maybe even before it is blocking other processes or stoped by oom killer?
MongoDB does not use its own memory management. Instead it uses the OS' LRU. The OS is paging documents so heavily because it has used the amount of memory allocated to mongod, aka your working set is bigger than the amount of RAM you have spare for MongoDB as such MongoDB is swapping page faults for most of it not all of your data ( a good reference for paging: http://en.wikipedia.org/wiki/Paging ).
I would strongly not recommend restricting MongoDB in this case since it will run even worse however, especially on Linux, you can actually use ulimit on the mongo user you are using to run mongod: http://docs.mongodb.org/manual/administration/ulimit/
Does MongoDB have any option to run a query or task in "safe-mode", e.g. without cutting the memory too much?
Not really.
It seems to be very touchy and can be crashed, e.g. by running some find queries with $lte/$gt on a non-indexed timestamp field.
Naturally this shouldn't cause an OOM exception for MongoDB, it could indicate a memory leak somewhere: http://docs.mongodb.org/manual/administration/ulimit/
If you limit the resident memory size on a system running MongoDB you risk allowing the operating system to terminate the mongod process under normal situations. Do not set this value. If the operating system (i.e. Linux) kills your mongod, with the OOM killer, check the output of serverStatus and ensure MongoDB is not leaking memory.

Answer 2

It seems to be very touchy and can be crashed e.g. by running some find queries with $lte/$gt on a non-indexed timestamp field.
It's the OOM killer that's killing it because your mongod instance is swapping a lot pages into RAM. You probably have a lot processes contending for RAM. You can instruct Linux to not kill the mongod daemon as follows :
sudo echo -17 > /proc/<process if of mongod>/oom_adj
You can't control how much memory mongodb uses, unfortunately. I suggest looking at the background indexing docs on mongodb. And some more useful links :
See the related thread on stackoverflow
How do i limit the cache size?