Erlang VM Tuning

Riak was written almost exclusively in Erlang and runs on an Erlang virtual machine (VM), which makes proper Erlang VM tuning an important part of optimizing Riak performance. The Erlang VM itself provides a wide variety of configurable parameters that you can use to tune its performance; Riak enables you to tune a subset of those parameters in each node’s configuration files.

The table below lists some of the parameters that are available, showing both their names as used in Erlang and their names as Riak parameters.

Erlang parameter Riak parameter
+A erlang.async_threads
+K erlang.K
+P erlang.process_limit
+Q erlang.max_ports
+W erlang.W
+a erlang.async_threads.stack_size
+e erlang.max_ets_tables
+scl erlang.schedulers.compaction_of_load
+sfwi erlang.schedulers.force_wakeup_interval
-smp erlang.smp
+sub erlang.schedulers.utilization_balancing
+zdbbl erlang.distribution_buffer_size
-kernel net_ticktime erlang.distribution.net_ticktime
-env FULLSWEEP_AFTER erlang.fullsweep_after
-env ERL_CRASH_DUMP erlang.crash_dump
-env ERL_MAX_ETS_TABLES erlang.max_ets_tables
-name nodename
Note on upgrading to 2.0

In versions of Riak prior to 2.0, Erlang VM-related parameters were specified in a vm.args configuration file; in versions 2.0 and later, all Erlang-VM-specific parameters are set in the riak.conf file. If you’re upgrading to 2.0 from an earlier version, you can still use your old vm.args if you wish. Please note, however, that if you set one or more parameters in both vm.args and in riak.conf, the settings in vm.args will override those in riak.conf.


Some operating systems provide Erlang VMs with Symmetric Multiprocessing capabilities (SMP) for taking advantage of multi-processor hardware architectures. SMP support can be turned on or off by setting the erlang.smp parameter to enable or disable. It is enabled by default. The following would disable SMP support:

erlang.smp = disable

Because Riak is supported on some operating systems that do not provide SMP support. Make sure that your OS supports SMP before enabling it for use by Riak’s Erlang VM. If it does not, you should set erlang.smp to disable prior to starting up your cluster.

Another safe option is to set erlang.smp to auto. This will instruct the Erlang VM to start up with SMP support enabled if (a) SMP support is available on the current OS and (b) more than one logical processor is detected. If neither of these conditions is met, the Erlang VM will start up with SMP disabled.


Note on missing scheduler flags

We recommend that all users set the +sfwi to 500 (milliseconds) and the +scl flag to false if using the older, vm.args-based configuration system. If you are using the new, riak.conf-based configuration system, the corresponding parameters are erlang.schedulers.force_wakeup_interval and erlang.schedulers.compaction_of_load.

Please note that you will need to uncomment the appropriate lines in your riak.conf for this configuration to take effect.

If SMP support has been enabled on your Erlang VM, i.e. if erlang.smp is set to enable or auto on a machine providing SMP support and more than one logical processor, you can configure the number of logical processors, or scheduler threads, that are created when starting Riak, as well as the number of threads that are set online.

The total number of threads can be set using the parameter, whereas the number of threads set online can be set using These parameters map directly onto Schedulers and SchedulersOnline, both of which are used by erl.

While the maximum for both parameters is 1024, there is no universal default for either. Instead, the Erlang VM will attempt to determine the number of configured processors, as well as the number of available processors, on its own. If the Erlang VM can make that determination, will default to the total number of configured processors while will default to the number of processors available; if the Erlang VM can’t make that determination, both values will default to 1.

If either parameter is set to a negative integer, that value will be subtracted from the default number of processors that are configured or available, depending on the parameter. For example, if there are 100 configured processors and is set to -50, then the calculated value for will be 50. Setting either parameter to 0, on the other hand, will reset both values to their defaults.

If SMP support is not enabled, i.e. if erlang.smp is set to disable (or set to auto on a machine without SMP support or with only one logical processor), then the values of and will be ignored.

Scheduler Wakeup Interval

Scheduler wakeup is an optional process whereby Erlang VM schedulers are periodically scanned to determine whether they have “fallen asleep,” i.e. whether they have an empty run queue. The interval at which this process occurs can be set, in milliseconds, using the erlang.schedulers.force_wakeup_interval parameter, which corresponds to the Erlang VM’s +sfwi flag. This parameter is set to 0 by default, which disables scheduler wakeup.

Erlang distributions like R15Bx have a tendency to put schedulers to sleep too often. If you are using a more recent distribution, i.e. a if you are running Riak 2.0 or later, you most likely won’t need to enable scheduler wakeup.

Scheduler Compaction and Balancing

The Erlang scheduler offers two methods of distributing load across schedulers: compaction of load and utilization balancing of load.

Compaction of load is used by default. When enabled, the Erlang VM will attempt to fully load as many scheduler threads as possible, i.e. it will attempt to ensure that scheduler threads do not run out of work. To that end, the VM will take into account the frequency with which schedulers run out of work when making decisions about which schedulers should be assigned work. You can disable compaction of load by setting the erlang.schedulers.compaction_of_load setting to false (in the older configuration system, set +scl to true).

The other option, utilization balancing, is disabled by default in favor of load balancing. When utilization balancing is enabled instead, the Erlang VM will strive to balance scheduler utilization as equally as possible between schedulers, without taking into account the frequency at which schedulers run out of work. You can enable utilization balancing by setting the erlang.schedulers.utilization_balancing setting to true (or the +scl parameter to false in the older configuration system).

At any given time, only compaction of load or utilization balancing can be used. If you set both parameters to false, Riak will default to using compaction of load; if both are set to true, Riak will enable whichever setting is listed first in riak.conf (or vm.args if you’re using the older configuration system).

Port Settings

Riak uses epmd, the Erlang Port Mapper Daemon, for most inter-node communication. In this system, other nodes in the cluster use the Erlang identifiers specified by the nodename parameter (or -name in vm.args), for example riak@ On each node, the daemon resolves these node identifiers to a TCP port. You can specify a port or range of ports for Riak nodes to listen on as well as the maximum number of concurrent ports/sockets.

Port Range

By default, epmd binds to TCP port 4369 and listens on the wildcard interface. epmd uses an unpredictable port for inter-node communication by default, binding to port 0, which means that it uses the first available port. This can make it difficult to configure firewalls.

To make configuring firewalls easier, you can instruct the Erlang VM to use either a limited range of TCP ports or a single TCP port. The minimum and maximum can be set using the erlang.distribution.port_range.minimum and erlang.distribution.port.maximum parameters, respectively. The following would set the range to ports between 3000 and 5000:

erlang.distribution.port_range.minimum = 3000
erlang.distribution.port_range.maximum = 5000
%% The older, app.config-based system uses different parameter names
%% for specifying the minimum and maximum port

{kernel, [
          % ...
          {inet_dist_listen_min, 3000},
          {inet_dist_listen_max, 5000}
          % ...

You can set the Erlang VM to use a single port by setting the minimum to the desired port while setting no maximum. The following would set the port to 5000:

erlang.distribution.port_range.minimum = 5000
{kernel, [
          % ...
          {inet_dist_listen_min, 5000},
          % ...

If the minimum port is unset, the Erlang VM will listen on a random high-numbered port.

Maximum Ports

You can set the maximum number of concurrent ports/sockets used by the Erlang VM using the erlang.max_ports setting. Possible values range from 1024 to 134217727. The default is 65536. In vm.args you can use either +Q or -env ERL_MAX_PORTS.

Asynchronous Thread Pool

If thread support is available in your Erlang VM, you can set the number of asynchronous threads in the Erlang VM’s asynchronous thread pool using erlang.async_threads (+A in vm.args). The valid range is 0 to 1024. If thread support is available on your OS, the default is 64. Below is an example setting the number of async threads to 600:

erlang.async_threads = 600
+A 600

Stack Size

In addition to the number of asynchronous threads, you can determine the memory allocated to each thread using the erlang.async_threads.stack_size parameter, which corresponds to the +a Erlang flag. You can determine that size in Riak using KB, MB, GB, etc. The valid range is 16-8192 kilowords, which translates to 64-32768 KB on 32-bit architectures. While there is no default, we suggest a stack size of 16 kilowords, which translates to 64 KB. We suggest such a small size because the number of asynchronous threads, as determined by erlang.async_threads might be quite large in your Erlang VM. The 64 KB default is enough for drivers delivered with Erlang/OTP but might not be large enough to accommodate drivers that use the driver_async() functionality, documented here. We recommend setting higher values with caution, always keeping the number of available threads in mind.

Kernel Polling

You can utilize kernel polling in your Erlang distribution if your OS supports it. Kernel polling can improve performance if many file descriptors are in use; the more file descriptors, the larger an effect kernel polling may have on performance. Kernel polling is enabled by default on Riak’s Erlang VM, i.e. the default for erlang.K is on. This corresponds to the +K setting on the Erlang VM. You can disable it by setting erlang.K to off.

Warning Messages

Erlang’s error_logger is an event manager that registers error, warning, and info events from the Erlang runtime. By default, events from the error_logger are mapped as warnings, but you can also set messages to be mapped as errors or info reports using the erlang.W parameter (or +W in vm.args). The possible values are w (warnings), errors, or i (info reports).

Process Limit

The erlang.process_limit parameter can be used to set the maximum number of simultaneously existing system processes (corresponding to Erlang’s +P parameter). The valid range is 1024 to 134217727. The default is 256000.

Distribution Buffer

You can set the size of the Erlang VM’s distribution buffer busy limit (denoted by +zdbbl on the VM and in vm.args) using erlang.distribution_buffer_size. Modifying this setting can be useful on nodes with many busy_dist_port events, i.e. instances when the Erlang distribution is overloaded. The default is 32 MB (i.e. 32MB), but this may be insufficient for some workloads. The maximum value is 2097151 KB.

A larger buffer limit will allow processes to buffer more outgoing messages. When the limit is reached, sending processes will be suspended until the the buffer size has shrunk below the limit specified by erlang.distribution_buffer_size. Higher values will tend to produce lower latency and higher throughput but at the expense of higher RAM usage. You should evaluate your RAM resources prior to increasing this setting.

Erlang Built-in Storage

Erlang uses a built-in database called ets (Erlang Term Storage) for some processes that require fast access from memory in constant access time (rather than logarithmic access time). The maximum number of tables can be set using the erlang.max_ets_tables setting. The default is 256000, which is higher than the default limit of 1400 on the Erlang VM. The corresponding setting in vm.args is +e.

Higher values for erlang.max_ets_tables will tend to provide more quick-access data storage but at the cost of higher RAM usage. Please note that the default values for erlang.max_ets_tables and erlang.distribution_size (explained in the section above) are the same.

Crash Dumps

By default, crash dumps from Riak’s Erlang distribution are deposited in ./log/erl_crash.dump. You can change this location using erlang.crash_dump. This is the equivalent of setting the ERL_CRASH_DUMP environment variable for the Erlang VM.

Net Kernel Tick Time

The net kernel is an Erlang system process that provides various forms of network monitoring. In a Riak cluster, one of the functions of the net kernel is to periodically check node liveness. Tick time is the frequency with which those checks happen. You can determine that frequency using the erlang.distribution.net_ticktime. The tick will occur every N seconds, where N is the value set. Thus, setting erlang.distribution.net_ticktime to 60 will make the tick occur once every minute. The corresponding flag in vm.args is -kernel net_ticktime.

Shutdown Time

You can determine how long the Erlang VM spends shutting down using the erlang.shutdown_time parameter. The default is 10s (10 seconds). Once this duration elapses, all existing processes are killed. Decreasing shutdown time can be useful in situations in which you are frequently starting and stopping a cluster, e.g. in test clusters. In vm.args you can set the -shutdown_time flag in milliseconds.