I am using MongoDB-2.6.1. Is there any way to limit the RAM memory usage?
For example: Out of 4 GB RAM, let MongoDB use till 3 GB (<= 3 GB).
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IIRC MongoDB doesn't directly use memory for data, it uses memory mapped files and lets the OS manage how much is loaded via the normal VFS behaviour. Changing per-process limits is unlikely to affect this.
You don't state what platform you are using at all, but if you are already using virtualisation could you could run Mongo in its own VM with a RAM allocation set to the memory you want Mongo to use plus some extra for the guest OS. You could also start using virtualisation for this reason but be aware there is likely to be an efficiency hit switching from a physical box to a VM.
If you are running on Linux, you can use control groups to limit MongoDB memory as shown in the following article:
Easy Steps to Limit Mongodb Memory Usage by Ramakanta Sahoo
On Windows, a similar technique using the Windows System Resource Manager is described in:
Working only with MongoDB, the answer to your question is no.
What operating system are you using? Why do you want to limit Mongo's RAM usage?
Answer from user1472187 will work, but unfortunately these settings won’t last a reboot. You need to add some init scripts: http://frank2.net/cgroups-ubuntu-14-04/
Shell script is available for Ubuntu 14.04: http://brainsuckerna.blogspot.com.by/2016/05/limiting-mongodb-memory-usage-with.html