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# System Limits

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The Erlang language specification puts no limits on the number of processes, length of atoms, and so on. However, for performance and memory saving reasons, there will always be limits in a practical implementation of the Erlang language and execution environment.

- Processes - The maximum number of simultaneously alive Erlang processes is by default 1,048,576. This limit can be configured at startup. For more information, see the `+P` command-line flag in the `erl(1)` manual page in ERTS.

- Unique Local Process Identifiers on a Runtime System Instance - On a 64 bit system at most `2⁶⁰ - 1` unique process identifiers can be created, and on a 32 bit system at most `2²⁸ - 1`.

- Known nodes - A remote node Y must be known to node X if there exists any pids, ports, references, or funs (Erlang data types) from Y on X, or if X and Y are connected. The maximum number of remote nodes simultaneously/ever known to a node is limited by the maximum number of atoms available for node names. All data concerning remote nodes, except for the node name atom, are garbage-collected.

- Connected nodes - The maximum number of simultaneously connected nodes is limited by either the maximum number of simultaneously known remote nodes, the maximum number of (Erlang) ports available, or the maximum number of sockets available.

- Characters in an atom - 255.

- Atoms - By default, the maximum number of atoms is 1,048,576. This limit can be raised or lowered using the `+t` option.

- Elements in a tuple - The maximum number of elements in a tuple is 16,777,215 (24-bit unsigned integer).

- Size of binary - In the 32-bit run-time system for Erlang, 536,870,911 bytes is the largest binary that can be constructed or matched using the bit syntax. In the 64-bit run-time system, the maximum size is 2,305,843,009,213,693,951 bytes. If the limit is exceeded, bit syntax construction fails with a `system_limit` exception, while any attempt to match a binary that is too large fails. From Erlang/OTP 27, all other operations that create binaries (such as `list_to_binary/1`) also enforces the same limit.

- Total amount of data allocated by an Erlang node - The Erlang runtime system can use the complete 32-bit (or 64-bit) address space, but the operating system often limits a single process to use less than that.

- Length of a node name - An Erlang node name has the form `host@shortname` or `host@longname`. The node name is used as an atom within the system, so the maximum size of 255 holds also for the node name.

- Open ports - The maximum number of simultaneously open Erlang ports is often by default 16,384. This limit can be configured at startup. For more information, see the `+Q` command-line flag in the `erl(1)` manual page in ERTS.

- Unique Local Port Identifiers on a Runtime System Instance - On a 64 bit system at most `2⁶⁰ - 1` unique port identifiers can be created and on a 32 bit system at most `2²⁸ - 1`.

- Open files and sockets - The maximum number of simultaneously open files and sockets depends on the maximum number of Erlang ports available, as well as on operating system-specific settings and limits.

- Number of arguments to a function or fun - 255.

- Unique References on a Runtime System Instance - Each scheduler thread has its own set of references, and all other threads have a shared set of references. Each set of references consist of `2⁶⁴ - 1`unique references. That is, the total amount of unique references that can be produced on a runtime system instance is `(NumSchedulers + 1) × (2⁶⁴ - 1)`. If a scheduler thread create a new reference each nano second, references will at earliest be reused after more than 584 years. That is, for the foreseeable future they are sufficiently unique.

- Unique Integers on a Runtime System Instance - There are two types of unique integers created by the erlang:unique_integer/1 BIF:

  - Unique integers created with the `monotonic` modifier consist of a set of `2⁶⁴ - 1` unique integers.
  - Unique integers created without the `monotonic` modifier consist of a set of `2⁶⁴ - 1` unique integers per scheduler thread and a set of `2⁶⁴ - 1` unique integers shared by other threads. That is, the total amount of unique integers without the `monotonic` modifier is `(NumSchedulers + 1) × (2⁶⁴ - 1)`.

  If a unique integer is created each nano second, unique integers will be reused at earliest after more than 584 years. That is, for the foreseeable future they are sufficiently unique.

- Timer resolution - On most systems, millisecond resolution. For more information, see the Timers section of the Time and Time Correction in Erlang ERTS User's guide.

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