I’ve seen the question of how to control MongoDB’s memory usage on Windows come up several times and the stock answer always seemed to be “you can’t – it uses memory-mapped files and if you want to limit resources you need to use some form of virtualization to do it (HyperV, VMWare, Virtuozzo etc…)”.

If you are using MongoDB on a dedicated server then you generally want it to use all the memory it can but if you want to use it on a server shared with other processes (e.g. an IIS website using MongoDB for storage, maybe with SQL Server as well) then you will want to put a cap on how much it uses to ensure memory is kept available for the other processes.

So is it possible if you are not on a virtualized environment? Yes (otherwise this would be a very short blog post!) and we’ll explore how …

The standard behaviour described above is actually a result of the default resource manager used by Windows but both Windows 2003 and Windows 2008 have a separate installable option called the “Windows System Resource Manager” (WSRM) that allows greater control over the CPU and Memory available to a process.

First of all, lets look at what we’re trying to solve. Here we have a low-memory server (only 2Gb) running MongoDB on Windows 2008 R2 x64. There are a few databases of a few Gb each so the mongod.exe process quickly starts consuming as much memory as it can (rightly so) to keep as much of it’s indexes in memory for the fast performance we know and love:

What we’d like to do is save some memory for other processes by limiting the mongod.exe process to 1Gb in this case (I know this is ridiculously low but the only thing that will change for you are the actual limits you want to use).

To do this we first need to install Windows System Resource Manager which on Windows 2008 is available under the Features section of the Server Manager.

Once that’s installed fire it up and you’ll see the default resource management policies. By default the standard Windows “memory is given to whoever shouts loudest” policy is used but other pre-configured alternatives are available. WSRM also provides a calendar / event system where the policy can be changed at certain times (a typical scenario is giving critical business apps priority during the day but then batch processes greater priority overnight). We’re not going to go into the calendar features here but it’s interesting to know about.

Let’s create a new policy to control the resources that MongoDB can consume. To do this, right click on the “Resource Allocation Policies” container and chose “New Resource Allocation Policy …”. This will present us with the New Resource Allocation Policy dialog below:

First of all, we need to add a new resource allocation entry so click the ‘Add…’ button and we get to another “Add or Edit Resource Allocation” dialog:

We don’t have a Process matching criteria for MongoDB yet so choose <New…> to get … yes, you guessed – another dialog, this time “New Process Matching Criteria”. We’ll call it “mongod_process” and click the Add… button to get another death-by-dialog to define it.

There are a few ways to do this – if MongoDB is installed as a service then you can choose “Registered Service” in the drop-down, click “Select” and choose it from the list or you can select from a list of running processes or you can just enter the full path and filename to mongod.exe. Here is the entry after selecting an installed MongoDB Windows Service:

After clicking OK we get back to the Process Matching Criteria dialog showing our new rule: