ERL(1) General Commands Manual ERL(1)

erl - Start the Erlang runtime system.

The erl program starts an Erlang runtime system. The exact details (for example, whether erl is a script or a program and which other programs it calls) are system-dependent.

Note
If you are using Erlang/OTP 25 or earlier on Windows and want to start an Erlang system with full shell support, you should use werl.exe. See the Erlang/OTP 25 documentation https://www.erlang.org/docs/25/man/werl for details on how to do that.

Starts an Erlang runtime system.

The arguments can be divided into emulator flags, flags, and plain arguments:

•
Any argument starting with character + is interpreted as an emulator flag.
As indicated by the name, emulator flags control the behavior of the emulator.
•
Any argument starting with character - (hyphen) is interpreted as a flag, which is to be passed to the Erlang part of the runtime system, more specifically to the init system process, see m:init.
The init process itself interprets some of these flags, the init flags. It also stores any remaining flags, the user flags. The latter can be retrieved by calling init:get_argument/1.
A small number of "-" flags exist, which now actually are emulator flags, see the description below.
•
Plain arguments are not interpreted in any way. They are also stored by the init process and can be retrieved by calling init:get_plain_arguments/0. Plain arguments can occur before the first flag, or after a -- flag. Also, the -extra flag causes everything that follows to become plain arguments.

Examples:

% erl +W w -sname arnie +R 9 -s my_init -extra +bertie
(arnie@host)1> init:get_argument(sname).
{ok,[["arnie"]]}
(arnie@host)2> init:get_plain_arguments().
["+bertie"]

Here +W w and +R 9 are emulator flags. -s my_init is an init flag, interpreted by init. -sname arnie is a user flag, stored by init. It is read by Kernel and causes the Erlang runtime system to become distributed. Finally, everything after -extra (that is, +bertie) is considered as plain arguments.

% erl -myflag 1
1> init:get_argument(myflag).
{ok,[["1"]]}
2> init:get_plain_arguments().
[]

Here the user flag -myflag 1 is passed to and stored by the init process. It is a user-defined flag, presumably used by some user-defined application.

In the following list, init flags are marked "(init flag)". Unless otherwise specified, all other flags are user flags, for which the values can be retrieved by calling init:get_argument/1. Notice that the list of user flags is not exhaustive, there can be more application-specific flags that instead are described in the corresponding application documentation.

The file FileName is to be a plain text file and can contain comments and command-line arguments. A comment begins with a # character and continues until the next end of line character. Backslash (\) is used as quoting character. All command-line arguments accepted by erl are allowed, also flag -args_file FileName. Be careful not to cause circular dependencies between files containing flag -args_file, though.
The flag -extra is treated in special way. Its scope ends at the end of the file. Arguments following an -extra flag are moved on the command line into the -extra section, that is, the end of the command line following after an -extra flag.
Defaults to $ROOT/bin/start.boot.
Not recommended; use erlc instead.
A configuration file descriptor will be read until its end and will then be closed.
The content of a configuration file descriptor is stored so that it can be reused when init:restart/0 or init:restart/1 is called.
The parameter -configfd 0 implies -noinput.
Note
It is not recommended to use file descriptors 1 (standard output), and 2 (standard error) together with -configfd as these file descriptors are typically used to print information to the console the program is running in.
Examples (Unix shell):
$ erl \
-noshell \
-configfd 3 \
-eval \
<(echo '[{kernel, [{logger_level, warning}]}].')
{ok,warning}
io:format("~p~n",[application:get_env(kernel, logger_level)]),erlang:halt()' 3< \
<(echo '[{kernel, [{logger_level, warning}]}].')
{ok,warning}
$ echo '[{kernel, [{logger_level, warning}]}].' > test1.config
$ echo '[{kernel, [{logger_level, error}]}].' > test2.config
$ erl \
-noshell \
-configfd 3 \
-configfd 4 \
-eval \
3< test1.config 4< test2.config
{ok,error}
io:format("~p~n",[application:get_env(kernel, logger_level)]),erlang:halt()' \
3< test1.config 4< test2.config
{ok,error}

% erl -env DISPLAY gin:0
In this example, an Erlang runtime system is started with environment variable DISPLAY set to gin:0.
The IP addresses must be specified in the standard form (four decimal numbers separated by periods, for example, "150.236.20.74"). Hosts names are not acceptable, but a broadcast address (preferably limited to the local network) is.
If Loader is something else, the user-supplied Loader port program is started.
The node name will be Name@Host, where Host is the fully qualified host name of the current host. For short names, use flag -sname instead.
If Name is set to undefined the node will be started in a special mode optimized to be the temporary client of another node. The node will then request a dynamic node name from the first node it connects to. Read more in Dynamic Node Name.
Warning
Starting a distributed node without also specifying -proto_dist inet_tls will expose the node to attacks that may give the attacker complete access to the node and in extension the cluster. When using un-secure distributed nodes, make sure that the network is configured to keep potential attackers out.

•
-no_epmd - Specifies that the distributed node does not need epmd at all.
This option ensures that the Erlang runtime system does not start epmd and does not start the m:erl_epmd process for distribution either.
This option only works if Erlang is started as a distributed node with the -proto_dist option using an alternative protocol for Erlang distribution which does not rely on epmd for node registration and discovery. For more information, see How to implement an Alternative Carrier for the Erlang Distribution.
As an alternative to -pa, if several directories are to be prepended to the code path and the directories have a common parent directory, that parent directory can be specified in environment variable ERL_LIBS; see m:code.
For example, to start up IPv6 distributed nodes:
% erl -name test@ipv6node.example.com -proto_dist inet6_tcp

•
-remsh Node - Starts Erlang with a remote shell connected to Node.
If no -name or -sname is given the node will be started using -sname undefined. If Node does not contain a hostname, one is automatically taken from -name or -sname
Note
Before OTP-23 the user needed to supply a valid -sname or -name for -remsh to work. This is still the case if the target node is not running OTP-23 or later.
Note
The connecting node needs to have a proper shell with terminal emulation. This means that UNIX users must use an Erlang compiled with terminal capabilities and before Erlang/OTP 25 Windows users must use werl.

This is sometimes the only way to run distributed Erlang if the Domain Name System (DNS) is not running. No communication can exist between nodes running with flag -sname and those running with flag -name, as node names must be unique in distributed Erlang systems.
If Name is set to undefined the node will be started in a special mode optimized to be the temporary client of another node. The node will then request a dynamic node name from the first node it connects to. Read more in Dynamic Node Name.
Warning
Starting a distributed node without also specifying -proto_dist inet_tls will expose the node to attacks that may give the attacker complete access to the node and in extension the cluster. When using un-secure distributed nodes, make sure that the network is configured to keep potential attackers out.

•
-start_epmd true | false - Specifies whether Erlang should start epmd on startup. By default this is true, but if you prefer to start epmd manually, set this to false.
This only applies if Erlang is started as a distributed node, i.e. if -name or -sname is specified. Otherwise, epmd is not started even if -start_epmd true is given.
Note that a distributed node will fail to start if epmd is not running.
•
-version (emulator flag) - Makes the emulator print its version number. The same as erl +V.

erl invokes the code for the Erlang emulator (virtual machine), which supports the following flags:

If option c is used with oldshell on Unix, Ctrl-C will restart the shell process rather than interrupt it.
For backward compatibility, the boolean value can be omitted. This is interpreted as +c false.

•
+d - If the emulator detects an internal error (or runs out of memory), it, by default, generates both a crash dump and a core dump. The core dump is, however, not very useful as the content of process heaps is destroyed by the crash dump generation.
Option +d instructs the emulator to produce only a core dump and no crash dump if an internal error is detected.
Calling erlang:halt/1 with a string argument still produces a crash dump. On Unix systems, sending an emulator process a SIGUSR1 signal also forces a crash dump.
•
+dcg DecentralizedCounterGroupsLimit - Limits the number of decentralized counter groups used by decentralized counters optimized for update operations in the Erlang runtime system. By default, the limit is 256.
When the number of schedulers is less than or equal to the limit, each scheduler has its own group. When the number of schedulers is larger than the groups limit, schedulers share groups. Shared groups degrade the performance for updating counters while many reader groups degrade the performance for reading counters. So, the limit is a tradeoff between performance for update operations and performance for read operations. Each group consumes 64 bytes in each counter.
Note that a runtime system using decentralized counter groups benefits from binding schedulers to logical processors, as the groups are distributed better between schedulers with this option.
This option only affects decentralized counters used for the counters that are keeping track of the memory consumption and the number of terms in ETS tables of type ordered_set with the write_concurrency option activated.
For more information about Unicode filenames, see section Unicode Filenames in the STDLIB User's Guide. Notice that this value also applies to command-line parameters and environment variables (see section Unicode in Environment and Parameters in the STDLIB User's Guide).
•
+fnu[{w|i|e}] - The virtual machine works with filenames as if they are encoded using UTF-8 (or some other system-specific Unicode encoding). This is the default on operating systems that enforce Unicode encoding, that is, Windows MacOS X and Android.
The +fnu switch can be followed by w, i, or e to control how wrongly encoded filenames are to be reported:
Notice that file:read_link/1 always returns an error if the link points to an invalid filename.
For more information about Unicode filenames, see section Unicode Filenames in the STDLIB User's Guide. Notice that this value also applies to command-line parameters and environment variables (see section Unicode in Environment and Parameters in the STDLIB User's Guide).
•
+fna[{w|i|e}] - Selection between +fnl and +fnu is done based on the current locale settings in the OS. This means that if you have set your terminal for UTF-8 encoding, the filesystem is expected to use the same encoding for filenames. This is the default on all operating systems, except Android, MacOS X and Windows.
The +fna switch can be followed by w, i, or e. This has effect if the locale settings cause the behavior of +fnu to be selected; see the description of +fnu above. If the locale settings cause the behavior of +fnl to be selected, then w, i, or e have no effect.
For more information about Unicode filenames, see section Unicode Filenames in the STDLIB User's Guide. Notice that this value also applies to command-line parameters and environment variables (see section Unicode in Environment and Parameters in the STDLIB User's Guide).
A good way to check if more IO poll threads are needed is to use microstate accounting and see what the load of the IO poll thread is. If it is high it could be a good idea to add more threads.
If enabled, file descriptors that are frequently read may be moved to a special pollset used by scheduler threads. The objective is to reduce the number of system calls and thereby CPU load, but it can in some cases increase scheduling latency for individual file descriptor input events.
•
+JPcover true|false|function|function_counters|line|line_counters - Since: OTP 27.0
Enables or disables support for coverage when running with the JIT. Defaults to false.

•
+JPperf true|false|dump|map|fp|no_fp - Enables or disables support for the perf profiler when running with the JIT on Linux. Defaults to false.
This option can be combined multiple times to enable several options:
For more details about how to run perf see the perf support section in the BeamAsm internal documentation.
•
+JMsingle true|false - Since: OTP-26.0
Enables or disables the use of single-mapped RWX memory for JIT code. The default is to map JIT:ed machine code into two regions sharing the same physical pages, where one region is executable but not writable, and the other writable but not executable. As some tools, such as QEMU user mode emulation, cannot deal with the dual mapping, this flags allows it to be disabled. This flag is automatically enabled by the +JPperf flag.
The boolean value used with the +pad parameter determines the default value of the async_dist process flag of newly spawned processes. By default, if no +pad command line option is passed, the async_dist flag will be set to false.
The value used in runtime can be inspected by calling erlang:system_info(async_dist).
•
+pc Range - Sets the range of characters that the system considers printable in heuristic detection of strings. This typically affects the shell, debugger, and io:format functions (when ~tp is used in the format string).
Two values are supported for Range:
See also io:printable_range/0 in STDLIB.
•
+P Number - Sets the maximum number of simultaneously existing processes for this system if a Number is passed as value. Valid range for Number is [1024-134217727].
Note
The actual maximum chosen may be much larger than the Number passed. Currently the runtime system often, but not always, chooses a value that is a power of 2. This might, however, be changed in the future. The actual value chosen can be checked by calling erlang:system_info(process_limit).
The default value is 1048576
•
+Q Number - Sets the maximum number of simultaneously existing ports for this system if a Number is passed as value. Valid range for Number is [1024-134217727].
Note
The actual maximum chosen may be much larger than the actual Number passed. Currently the runtime system often, but not always, chooses a value that is a power of 2. This might, however, be changed in the future. The actual value chosen can be checked by calling erlang:system_info(port_limit).
The default value used is normally 65536. However, if the runtime system is able to determine maximum amount of file descriptors that it is allowed to open and this value is larger than 65536, the chosen value will increased to a value larger or equal to the maximum amount of file descriptors that can be opened.
On Windows the default value is set to 8196 because the normal OS limitations are set higher than most machines can handle.
•
+R ReleaseNumber - Sets the compatibility mode.
The distribution mechanism is not backward compatible by default. This flag sets the emulator in compatibility mode with an earlier Erlang/OTP release ReleaseNumber. The release number must be in the range <current release>-2 through <current release>. This limits the emulator, making it possible for it to communicate with Erlang nodes (as well as C and Java nodes) running that earlier release.
Note
Ensure that all nodes (Erlang-, C-, and Java nodes) of a distributed Erlang system is of the same Erlang/OTP release, or from two different Erlang/OTP releases X and Y, where all Y nodes have compatibility mode X.

When the number of schedulers is less than or equal to the reader groups limit, each scheduler has its own reader group. When the number of schedulers is larger than the reader groups limit, schedulers share reader groups. Shared reader groups degrade read lock and read unlock performance while many reader groups degrade write lock performance. So, the limit is a tradeoff between performance for read operations and performance for write operations. Each reader group consumes 64 byte in each read/write lock.
Notice that a runtime system using shared reader groups benefits from binding schedulers to logical processors, as the reader groups are distributed better between schedulers.
•
+S Schedulers:SchedulerOnline - Sets the number of scheduler threads to create and scheduler threads to set online. The maximum for both values is 1024. If the Erlang runtime system is able to determine the number of logical processors configured and logical processors available, Schedulers defaults to logical processors configured, and SchedulersOnline defaults to logical processors available; otherwise the default values are 1. If the emulator detects that it is subject to a CPU quota, the default value for SchedulersOnline will be limited accordingly.
Schedulers can be omitted if :SchedulerOnline is not and conversely. The number of schedulers online can be changed at runtime through erlang:system_flag(schedulers_online, SchedulersOnline).
If Schedulers or SchedulersOnline is specified as a negative number, the value is subtracted from the default number of logical processors configured or logical processors available, respectively.
Specifying value 0 for Schedulers or SchedulersOnline resets the number of scheduler threads or scheduler threads online, respectively, to its default value.
•
+SP SchedulersPercentage:SchedulersOnlinePercentage - Similar to +S but uses percentages to set the number of scheduler threads to create, based on logical processors configured, and scheduler threads to set online, based on logical processors available. Specified values must be > 0. For example, +SP 50:25 sets the number of scheduler threads to 50% of the logical processors configured, and the number of scheduler threads online to 25% of the logical processors available. SchedulersPercentage can be omitted if :SchedulersOnlinePercentage is not and conversely. The number of schedulers online can be changed at runtime through erlang:system_flag(schedulers_online, SchedulersOnline).
This option interacts with +S settings. For example, on a system with 8 logical cores configured and 8 logical cores available, the combination of the options +S 4:4 +SP 50:25 (in either order) results in 2 scheduler threads (50% of 4) and 1 scheduler thread online (25% of 4).
For details, see +S and +SP. By default, the number of dirty CPU scheduler threads created equals the number of normal scheduler threads created, and the number of dirty CPU scheduler threads online equals the number of normal scheduler threads online. DirtyCPUSchedulers can be omitted if :DirtyCPUSchedulersOnline is not and conversely. The number of dirty CPU schedulers online can be changed at runtime through erlang:system_flag(dirty_cpu_schedulers_online, DirtyCPUSchedulersOnline).
The amount of dirty CPU schedulers is limited by the amount of normal schedulers in order to limit the effect on processes executing on ordinary schedulers. If the amount of dirty CPU schedulers was allowed to be unlimited, dirty CPU bound jobs would potentially starve normal jobs.
Typical users of the dirty CPU schedulers are large garbage collections, json protocol encode/decoders written as nifs and matrix manipulation libraries.
You can use m:msacc in order to see the current load of the dirty CPU schedulers threads and adjust the number used accordingly.
•
+SDPcpu DirtyCPUSchedulersPercentage:DirtyCPUSchedulersOnlinePercentage - Similar to +SDcpu but uses percentages to set the number of dirty CPU scheduler threads to create and the number of dirty CPU scheduler threads to set online. Specified values must be > 0. For example, +SDPcpu 50:25 sets the number of dirty CPU scheduler threads to 50% of the logical processors configured and the number of dirty CPU scheduler threads online to 25% of the logical processors available. DirtyCPUSchedulersPercentage can be omitted if :DirtyCPUSchedulersOnlinePercentage is not and conversely. The number of dirty CPU schedulers online can be changed at runtime through erlang:system_flag(dirty_cpu_schedulers_online, DirtyCPUSchedulersOnline).
This option interacts with +SDcpu settings. For example, on a system with 8 logical cores configured and 8 logical cores available, the combination of the options +SDcpu 4:4 +SDPcpu 50:25 (in either order) results in 2 dirty CPU scheduler threads (50% of 4) and 1 dirty CPU scheduler thread online (25% of 4).
•
+SDio DirtyIOSchedulers - Sets the number of dirty I/O scheduler threads to create. Valid range is 1-1024. By default, the number of dirty I/O scheduler threads created is 10.
The amount of dirty IO schedulers is not limited by the amount of normal schedulers like the amount of dirty CPU schedulers. This since only I/O bound work is expected to execute on dirty I/O schedulers. If the user should schedule CPU bound jobs on dirty I/O schedulers, these jobs might starve ordinary jobs executing on ordinary schedulers.
Typical users of the dirty IO schedulers are reading and writing to files.
You can use m:msacc in order to see the current load of the dirty IO schedulers threads and adjust the number used accordingly.
Schedulers can also be bound using flag +stbt. The only difference between these two flags is how the following errors are handled:
If any of these errors occur when +sbt has been passed, the runtime system prints an error message, and refuses to start. If any of these errors occur when +stbt has been passed, the runtime system silently ignores the error, and start up using unbound schedulers.
Valid BindTypes:
Binding of schedulers is only supported on newer Linux, Solaris, FreeBSD, and Windows systems.
If no CPU topology is available when flag +sbt is processed and BindType is any other type than u, the runtime system fails to start. CPU topology can be defined using flag +sct. Notice that flag +sct can have to be passed before flag +sbt on the command line (if no CPU topology has been automatically detected).
The runtime system does by default not bind schedulers to logical processors.
Note
If the Erlang runtime system is the only operating system process that binds threads to logical processors, this improves the performance of the runtime system. However, if other operating system processes (for example another Erlang runtime system) also bind threads to logical processors, there can be a performance penalty instead. This performance penalty can sometimes be severe. If so, you are advised not to bind the schedulers.
How schedulers are bound matters. For example, in situations when there are fewer running processes than schedulers online, the runtime system tries to migrate processes to schedulers with low scheduler identifiers. The more the schedulers are spread over the hardware, the more resources are available to the runtime system in such situations.
Note
If a scheduler fails to bind, this is often silently ignored, as it is not always possible to verify valid logical processor identifiers. If an error is reported, it is reported to the error_logger. If you want to verify that the schedulers have bound as requested, call erlang:system_info(scheduler_bindings).

•
+sbwt none|very_short|short|medium|long|very_long - Sets scheduler busy wait threshold. Defaults to medium. The threshold determines how long schedulers are to busy wait when running out of work before going to sleep.
Note
This flag can be removed or changed at any time without prior notice.

•
+sbwtdcpu none|very_short|short|medium|long|very_long - As +sbwt but affects dirty CPU schedulers. Defaults to short.
Note
This flag can be removed or changed at any time without prior notice.

•
+sbwtdio none|very_short|short|medium|long|very_long - As +sbwt but affects dirty IO schedulers. Defaults to short.
Note
This flag can be removed or changed at any time without prior notice.

•
+scl true|false - Enables or disables scheduler compaction of load. By default scheduler compaction of load is enabled. When enabled, load balancing strives for a load distribution, which causes as many scheduler threads as possible to be fully loaded (that is, not run out of work). This is accomplished by migrating load (for example, runnable processes) into a smaller set of schedulers when schedulers frequently run out of work. When disabled, the frequency with which schedulers run out of work is not taken into account by the load balancing logic.
+scl false is similar to +sub true, but +sub true also balances scheduler utilization between schedulers.
•
+sct CpuTopology - Sets a user-defined CPU topology. The user-defined CPU topology overrides any automatically detected CPU topology. The CPU topology is used when binding schedulers to logical processors.
<Id> = integer(); when 0 =< <Id> =< 65535
<IdRange> = <Id>-<Id>
<IdOrIdRange> = <Id> | <IdRange>
<IdList> = <IdOrIdRange>,<IdOrIdRange> | <IdOrIdRange>
<LogicalIds> = L<IdList>
<ThreadIds> = T<IdList> | t<IdList>
<CoreIds> = C<IdList> | c<IdList>
<ProcessorIds> = P<IdList> | p<IdList>
<NodeIds> = N<IdList> | n<IdList>
<IdDefs> = <LogicalIds><ThreadIds><CoreIds><ProcessorIds><NodeIds> |
           <LogicalIds><ThreadIds><CoreIds><NodeIds><ProcessorIds>
CpuTopology = <IdDefs>:<IdDefs> | <IdDefs>
Uppercase letters signify real identifiers and lowercase letters signify fake identifiers only used for description of the topology. Identifiers passed as real identifiers can be used by the runtime system when trying to access specific hardware; if they are incorrect the behavior is undefined. Faked logical CPU identifiers are not accepted, as there is no point in defining the CPU topology without real logical CPU identifiers. Thread, core, processor, and node identifiers can be omitted. If omitted, the thread ID defaults to t0, the core ID defaults to c0, the processor ID defaults to p0, and the node ID is left undefined. Either each logical processor must belong to only one NUMA node, or no logical processors must belong to any NUMA nodes.
Both increasing and decreasing <IdRange>s are allowed.
NUMA node identifiers are system wide. That is, each NUMA node on the system must have a unique identifier. Processor identifiers are also system wide. Core identifiers are processor wide. Thread identifiers are core wide.
The order of the identifier types implies the hierarchy of the CPU topology. The valid orders are as follows:
A CPU topology can consist of both processor external, and processor internal NUMA nodes as long as each logical processor belongs to only one NUMA node. If <ProcessorIds> is omitted, its default position is before <NodeIds>. That is, the default is processor external NUMA nodes.
If a list of identifiers is used in an <IdDefs>:
A simple example. A single quad core processor can be described as follows:
% erl +sct L0-3c0-3
1> erlang:system_info(cpu_topology).
[{processor,[{core,{logical,0}},
             {core,{logical,1}},
             {core,{logical,2}},
             {core,{logical,3}}]}]
A more complicated example with two quad core processors, each processor in its own NUMA node. The ordering of logical processors is a bit weird. This to give a better example of identifier lists:
% erl +sct L0-1,3-2c0-3p0N0:L7,4,6-5c0-3p1N1
1> erlang:system_info(cpu_topology).
[{node,[{processor,[{core,{logical,0}},
                    {core,{logical,1}},
                    {core,{logical,3}},
                    {core,{logical,2}}]}]},
 {node,[{processor,[{core,{logical,7}},
                    {core,{logical,4}},
                    {core,{logical,6}},
                    {core,{logical,5}}]}]}]
As long as real identifiers are correct, it is OK to pass a CPU topology that is not a correct description of the CPU topology. When used with care this can be very useful. This to trick the emulator to bind its schedulers as you want. For example, if you want to run multiple Erlang runtime systems on the same machine, you want to reduce the number of schedulers used and manipulate the CPU topology so that they bind to different logical CPUs. An example, with two Erlang runtime systems on a quad core machine:
% erl +sct L0-3c0-3 +sbt db +S3:2 -detached -noinput -noshell -sname one
% erl +sct L3-0c0-3 +sbt db +S3:2 -detached -noinput -noshell -sname two
In this example, each runtime system have two schedulers each online, and all schedulers online will run on different cores. If we change to one scheduler online on one runtime system, and three schedulers online on the other, all schedulers online will still run on different cores.
Notice that a faked CPU topology that does not reflect how the real CPU topology looks like is likely to decrease the performance of the runtime system.
For more information, see erlang:system_info(cpu_topology).
•
+ssrct - Skips reading CPU topology.
Note
Reading CPU topology slows down startup when starting many parallel instances of ERTS on systems with large amount of cores; using this flag might speed up execution in such scenarios.

•
+sfwi Interval - Sets scheduler-forced wakeup interval. All run queues are scanned each Interval milliseconds. While there are sleeping schedulers in the system, one scheduler is woken for each non-empty run queue found. Interval default to 0, meaning this feature is disabled.
Note
This feature has been introduced as a temporary workaround for long-executing native code, and native code that does not bump reductions properly in OTP. When these bugs have been fixed, this flag will be removed.

+sub true is only supported on systems where the runtime system detects and uses a monotonically increasing high-resolution clock. On other systems, the runtime system fails to start.
+sub true implies +scl false. The difference between +sub true and +scl false is that +scl false does not try to balance the scheduler utilization.
•
+swct very_eager|eager|medium|lazy|very_lazy - Sets scheduler wake cleanup threshold. Defaults to medium. Controls how eager schedulers are to be requesting wakeup because of certain cleanup operations. When a lazy setting is used, more outstanding cleanup operations can be left undone while a scheduler is idling. When an eager setting is used, schedulers are more frequently woken, potentially increasing CPU-utilization.
Note
This flag can be removed or changed at any time without prior notice.

•
+sws default|legacy - Sets scheduler wakeup strategy. Default strategy changed in ERTS 5.10 (Erlang/OTP R16A). This strategy was known as proposal in Erlang/OTP R15. The legacy strategy was used as default from R13 up to and including R15.
Note
This flag can be removed or changed at any time without prior notice.

•
+swt very_low|low|medium|high|very_high - Sets scheduler wakeup threshold. Defaults to medium. The threshold determines when to wake up sleeping schedulers when more work than can be handled by currently awake schedulers exists. A low threshold causes earlier wakeups, and a high threshold causes later wakeups. Early wakeups distribute work over multiple schedulers faster, but work does more easily bounce between schedulers.
Note
This flag can be removed or changed at any time without prior notice.

•
+swtdcpu very_low|low|medium|high|very_high - As +swt but affects dirty CPU schedulers. Defaults to medium.
Note
This flag can be removed or changed at any time without prior notice.

•
+swtdio very_low|low|medium|high|very_high - As +swt but affects dirty IO schedulers. Defaults to medium.
Note
This flag can be removed or changed at any time without prior notice.

Modified timing affects the following:
Note
Performance suffers when modified timing is enabled. This flag is only intended for testing and debugging.
return_to and return_from trace messages are lost when tracing on the spawn BIFs.
This flag can be removed or changed at any time without prior notice.

A larger buffer limit allows processes to buffer more outgoing messages over the distribution. When the buffer limit has been reached, sending processes will be suspended until the buffer size has shrunk. The buffer limit is per distribution channel. A higher limit gives lower latency and higher throughput at the expense of higher memory use.
This limit only affects processes that have disabled _fully asynchronous distributed signaling_.
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+zdntgc time - Sets the delayed node table garbage collection time (delayed_node_table_gc) in seconds. Valid values are either infinity or an integer in the range 0-100000000. Defaults to 60.
Node table entries that are not referred linger in the table for at least the amount of time that this parameter determines. The lingering prevents repeated deletions and insertions in the tables from occurring.
If flushing during a halt operation has been ongoing for <timeout> milliseconds, the flushing will be interrupted and the runtime system will be immediately terminated with exit code 255. If halting without flushing, the <timeout> will have no effect on the system.
The value set by this flag can be read by Erlang code by calling erlang:system_info(halt_flush_timeout).
See also the flush_timeout option of the erlang:halt/2 BIF. Note that the shortest timeout of this command line argument and the flush_timeout option will be the actual timeout value in effect.
Since: OTP 27.0

See also m:heart.
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ERL_CRASH_DUMP_BYTES - This variable sets the maximum size of a crash dump file in bytes. The crash dump will be truncated if this limit is exceeded. If the variable is not set, no size limit is enforced by default. If the variable is set to 0, the runtime system does not even attempt to write a crash dump file.
Introduced in ERTS 8.1.2 (Erlang/OTP 19.2).
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ERL_AFLAGS - The content of this variable is added to the beginning of the command line for erl.
Flag -extra is treated in a special way. Its scope ends at the end of the environment variable content. Arguments following an -extra flag are moved on the command line into section -extra, that is, the end of the command line following an -extra flag.
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ERL_ZFLAGS and ERL_FLAGS - The content of these variables are added to the end of the command line for erl.
Flag -extra is treated in a special way. Its scope ends at the end of the environment variable content. Arguments following an -extra flag are moved on the command line into section -extra, that is, the end of the command line following an -extra flag.

On Unix systems, the Erlang runtime will interpret two types of signals.

Introduced in ERTS 8.3 (Erlang/OTP 19.3)

The signal SIGUSR2 is reserved for internal usage. No other signals are handled.

The standard Erlang/OTP system can be reconfigured to change the default behavior on startup.

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The .erlang startup file - When Erlang/OTP is started, the system searches for a file named .erlang in the user's home directory and then filename:basedir(user_config, "erlang").
If an .erlang file is found, it is assumed to contain valid Erlang expressions. These expressions are evaluated as if they were input to the shell.
A typical .erlang file contains a set of search paths, for example:
io:format("executing user profile in $HOME/.erlang\n",[]).
code:add_path("/home/calvin/test/ebin").
code:add_path("/home/hobbes/bigappl-1.2/ebin").
io:format(".erlang rc finished\n",[]).

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user_default and shell_default - Functions in the shell that are not prefixed by a module name are assumed to be functional objects (funs), built-in functions (BIFs), or belong to the module user_default or shell_default.
To include private shell commands, define them in a module user_default and add the following argument as the first line in the .erlang file:
code:load_abs("..../user_default").

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erl - If the contents of .erlang are changed and a private version of user_default is defined, the Erlang/OTP environment can be customized. More powerful changes can be made by supplying command-line arguments in the startup script erl. For more information, see m:init.

epmd(1), m:erl_prim_loader, erts_alloc(3), m:init, m:application, m:auth, m:code, m:erl_boot_server, m:heart, m:net_kernel, m:make

January 2025