User's Guide
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Efficiency Guide
User's Guide
Version 5.9.1

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Chapters

- Introduction
  - Top of chapter
  - Purpose
  - Prerequisites
- The Eight Myths of Erlang Performance
  - Top of chapter
  - Myth: Funs are slow
  - Myth: List comprehensions are slow
  - Myth: Tail-recursive functions are MUCH faster than recursive functions
  - Myth: '++' is always bad
  - Myth: Strings are slow
  - Myth: Repairing a Dets file is very slow
  - Myth: BEAM is a stack-based byte-code virtual machine (and therefore slow)
  - Myth: Use '_' to speed up your program when a variable is not used
- Common Caveats
  - Top of chapter
  - The timer module
  - list_to_atom/1
  - length/1
  - setelement/3
  - size/1
  - split_binary/2
  - The '--' operator
- Constructing and matching binaries
  - Top of chapter
  - How binaries are implemented
  - Constructing binaries
  - Matching binaries
- List handling
  - Top of chapter
  - Creating a list
  - List comprehensions
  - Deep and flat lists
  - Why you should not worry about recursive lists functions
- Functions
  - Top of chapter
  - Pattern matching
  - Function Calls
  - Memory usage in recursion
- Tables and databases
  - Top of chapter
  - Ets, Dets and Mnesia
  - Ets specific
  - Mnesia specific
- Processes
  - Top of chapter
  - Creation of an Erlang process
  - Process messages
  - The SMP emulator
- Drivers
  - Top of chapter
  - Drivers and concurrency
  - Avoiding copying of binaries when calling a driver
  - Returning small binaries from a driver
  - Returning big binaries without copying from a driver
- Advanced
  - Top of chapter
  - Memory
  - System limits
- Profiling
  - Top of chapter
  - Do not guess about performance - profile
  - Big systems
  - What to look for
  - Tools
  - Benchmarking

# 10 Advanced

### 10.1 Memory

A good start when programming efficiently is to have knowledge about how much memory different data types and operations require. It is implementation-dependent how much memory the Erlang data types and other items consume, but here are some figures for the erts-5.2 system (OTP release R9B). (There have been no significant changes in R13.)

The unit of measurement is memory words. There exists both a 32-bit and a 64-bit implementation, and a word is therefore, 4 bytes or 8 bytes, respectively.

Data type | Memory size
Small integer | 1 word On 32-bit architectures: -134217729 < i < 134217728 (28 bits) On 64-bit architectures: -576460752303423489 < i < 576460752303423488 (60 bits)
Big integer | 3..N words
Atom | 1 word. Note: an atom refers into an atom table which also consumes memory. The atom text is stored once for each unique atom in this table. The atom table is not garbage-collected.
Float | On 32-bit architectures: 4 words On 64-bit architectures: 3 words
Binary | 3..6 + data (can be shared)
List | 1 word + 1 word per element + the size of each element
String (is the same as a list of integers) | 1 word + 2 words per character
Tuple | 2 words + the size of each element
Pid | 1 word for a process identifier from the current local node, and 5 words for a process identifier from another node. Note: a process identifier refers into a process table and a node table which also consumes memory.
Port | 1 word for a port identifier from the current local node, and 5 words for a port identifier from another node. Note: a port identifier refers into a port table and a node table which also consumes memory.
Reference | On 32-bit architectures: 5 words for a reference from the current local node, and 7 words for a reference from another node. On 64-bit architectures: 4 words for a reference from the current local node, and 6 words for a reference from another node. Note: a reference refers into a node table which also consumes memory.
Fun | 9..13 words + size of environment. Note: a fun refers into a fun table which also consumes memory.
Ets table | Initially 768 words + the size of each element (6 words + size of Erlang data). The table will grow when necessary.
Erlang process | 327 words when spawned including a heap of 233 words.

Table 10.1: Memory size of different data types

### 10.2 System limits

The Erlang language specification puts no limits on number of processes, length of atoms etc., but 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 32768. This limit can be raised up to at most 268435456 processes at startup (see documentation of the system flag +P in the erl(1) documentation). The maximum limit of 268435456 processes will at least on a 32-bit architecture be impossible to reach due to memory shortage.

Distributed nodes
  Known nodes

    A remote node Y has to be known to node X if there exist 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 1048576. This limit can be raised or lowered using the +t option.
Ets-tables
  The default is 1400, can be changed with the environment variable ERL_MAX_ETS_TABLES.
Elements in a tuple
  The maximum number of elements in a tuple is 67108863 (26 bit unsigned integer). Other factors such as the available memory can of course make it hard to create a tuple of that size.
Size of binary
  In the 32-bit implementation of Erlang, 536870911 bytes is the largest binary that can be constructed or matched using the bit syntax. (In the 64-bit implementation, the maximum size is 2305843009213693951 bytes.) If the limit is exceeded, bit syntax construction will fail with a system_limit exception, while any attempt to match a binary that is too large will fail. This limit is enforced starting with the R11B-4 release; in earlier releases, operations on too large binaries would in general either fail or give incorrect results. In future releases of Erlang/OTP, other operations that create binaries (such as list_to_binary/1) will probably also enforce the same limit.
Total amount of data allocated by an Erlang node
  The Erlang runtime system can use the complete 32 (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 for the node name too.
Open ports

  The maximum number of simultaneously open Erlang ports is by default 1024. This limit can be raised up to at most 268435456 at startup (see environment variable ERL_MAX_PORTS in erlang(3)) The maximum limit of 268435456 open ports will at least on a 32-bit architecture be impossible to reach due to memory shortage.

Open files, and sockets
  The maximum number of simultaneously open files and sockets depend on the maximum number of Erlang ports available, and operating system specific settings and limits.
Number of arguments to a function or fun
  255