Memory Alarms

The RabbitMQ server detects the total amount of RAM installed in the computer on startup and when rabbitmqctl set_vm_memory_high_watermark fraction is executed. By default, when the RabbitMQ server uses above 40% of the installed RAM, it raises a memory alarm and blocks all connections. Once the memory alarm has cleared (e.g. due to the server paging messages to disk or delivering them to clients) normal service resumes.

The default memory threshold is set to 40% of installed RAM. Note that this does not prevent the RabbitMQ server from using more than 40%, it is merely the point at which publishers are throttled. Erlang's garbage collector can, in the worst case, cause double the amount of memory to be used (by default, 80% of RAM). It is strongly recommended that OS swap or page files are enabled.

32-bit architectures tend to impose a per process memory limit of 2GB. Common implementations of 64-bit architectures (i.e. AMD64 and Intel EM64T) permit only a paltry 256TB per process. 64-bit Windows further limits this to 8TB. However, note that even under 64-bit OSes, a 32-bit process frequently only has a maximum address space of 2GB.

Configuring the Memory Threshold

The memory threshold at which the flow control is triggered can be adjusted by editing the configuration file. The example below sets the threshold to the default value of 0.4:

[{rabbit, [{vm_memory_high_watermark, 0.4}]}].

The default value of 0.4 stands for 40% of installed RAM or 40% of available virtual address space, whichever is smaller. E.g. on a 32-bit platform, if you have 4GB of RAM installed, 40% of 4GB is 1.6GB, but 32-bit Windows normally limits processes to 2GB, so the threshold is actually to 40% of 2GB (which is 820MB).

The memory limit is appended to the RABBITMQ_NODENAME.log file when the RabbitMQ server starts:

=INFO REPORT==== 29-Oct-2009::15:43:27 ===
Memory limit set to 2048MB.
The memory limit may also be queried using the rabbitmqctl status command.

The threshold can be changed while the broker is running using the rabbitmqctl set_vm_memory_high_watermark fraction command. This command will take effect until the broker shuts down. The corresponding configuration setting should also be changed when the effects should survive a broker restart. The memory limit may change on systems with hot-swappable RAM when this command is executed without altering the threshold, due to the fact that the total amount of system RAM is queried.

Disabling all publishing

A value of 0 makes the memory alarm go off immediately and thus disables all publishing (this may be useful if you wish to disable publishing globally); use rabbitmqctl set_vm_memory_high_watermark 0.

Limited Address Space

When running RabbitMQ inside a 32 bit Erlang VM in a 64 bit OS (or a 32 bit OS with PAE), the addressable memory is limited. The server will detect this and log a message like:

=WARNING REPORT==== 19-Dec-2013::11:27:13 ===
Only 2048MB of 12037MB memory usable due to limited address space.
Crashes due to memory exhaustion are possible - see
http://www.rabbitmq.com/memory.html#address-space

The memory alarm system is not perfect. While stopping publishing will usually prevent any further memory from being used, it is quite possible for other things to continue to increase memory use. Normally when this happens and the physical memory is exhausted the OS will start to swap. But when running with a limited address space, running over the limit will cause the VM to crash.

It is therefore strongly recommended that when running on a 64 bit OS you use a 64 bit Erlang VM.

Configuring the Paging Threshold

Before the broker hits the high watermark and blocks publishers, it will attempt to free up memory by instructing queues to page their contents out to disc. Both persistent and transient messages will be paged out (the persistent messages will already be on disc but will be evicted from memory).

By default this starts to happen when the broker is 50% of the way to the high watermark (i.e. with a default high watermark of 0.4, this is when 20% of memory is used). To change this value, modify the vm_memory_high_watermark_paging_ratio configuration from its default value of 0.5. For example:

[{rabbit, [{vm_memory_high_watermark_paging_ratio, 0.75},
         {vm_memory_high_watermark, 0.4}]}].

The above configuration starts paging at 30% of memory used, and blocks publishers at 40%.

It is possible to set vm_memory_high_watermark_paging_ratio to a greater value than 1.0. In this case queues will not page their contents to disc. If this causes the memory alarm to go off, then producers will be blocked as explained above.

Unrecognised platforms

If the RabbitMQ server is unable to recognise your system, it will append a warning to the RABBITMQ_NODENAME.log file. It then assumes than 1GB of RAM is installed:

=WARNING REPORT==== 29-Oct-2009::17:23:44 ===
Unknown total memory size for your OS {unix,magic_homebrew_os}. Assuming memory size is 1024MB.

In this case, the vm_memory_high_watermark configuration value is used to scale the assumed 1GB RAM. With the default value of vm_memory_high_watermark set to 0.4, RabbitMQ's memory threshold is set to 410MB, thus it will throttle producers whenever RabbitMQ is using more than 410MB memory. Thus when RabbitMQ can't recognize your platform, if you actually have 8GB RAM installed and you want RabbitMQ to throttle producers when the server is using above 3GB, set vm_memory_high_watermark to 3.

It is advised you do not set the threshold above 50% of your installed RAM.