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<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<copyright>
<year>2001</year><year>2016</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
</legalnotice>
<title>Advanced</title>
<prepared>Kenneth Lundin</prepared>
<docno></docno>
<date>2001-08-21</date>
<rev></rev>
<file>advanced.xml</file>
</header>
<section>
<title>Memory</title>
<p>A good start when programming efficiently is to know
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 the following table shows some figures for
the <c>erts-8.0</c> system in OTP 19.0.</p>
<p>The unit of measurement is memory words. There exists both a
32-bit and a 64-bit implementation. A word is therefore 4 bytes or
8 bytes, respectively.</p>
<table>
<row>
<cell><em>Data Type</em></cell>
<cell><em>Memory Size</em></cell>
</row>
<row>
<cell>Small integer</cell>
<cell>1 word.<br></br>
On 32-bit architectures: -134217729 < i < 134217728
(28 bits).<br></br>
On 64-bit architectures: -576460752303423489 < i <
576460752303423488 (60 bits).</cell>
</row>
<row>
<cell>Large integer</cell>
<cell>3..N words.</cell>
</row>
<row>
<cell>Atom</cell>
<cell>1 word.<br></br>
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 <em>not</em> garbage-collected.</cell>
</row>
<row>
<cell>Float</cell>
<cell>On 32-bit architectures: 4 words.<br></br>
On 64-bit architectures: 3 words.</cell>
</row>
<row>
<cell>Binary</cell>
<cell>3..6 words + data (can be shared).</cell>
</row>
<row>
<cell>List</cell>
<cell>1 word + 1 word per element + the size of each element.</cell>
</row>
<row>
<cell>String (is the same as a list of integers)</cell>
<cell>1 word + 2 words per character.</cell>
</row>
<row>
<cell>Tuple</cell>
<cell>2 words + the size of each element.</cell>
</row>
<row>
<cell>Small Map</cell>
<cell>5 words + the size of all keys and values.</cell>
</row>
<row>
<cell>Large Map (> 32 keys)</cell>
<cell>
<c>N</c> x <c>F</c> words + the size of all keys and values.<br></br>
<c>N</c> is the number of keys in the Map.<br></br>
<c>F</c> is a sparsity factor that can vary between 1.6 and 1.8
due to the probabilistic nature of the internal HAMT data structure.
</cell>
</row>
<row>
<cell>Pid</cell>
<cell>1 word for a process identifier from the current local node
+ 5 words for a process identifier from another node.<br></br>
A process identifier refers into a process table and a node table,
which also consumes memory.</cell>
</row>
<row>
<cell>Port</cell>
<cell>1 word for a port identifier from the current local node
+ 5 words for a port identifier from another node.<br></br>
A port identifier refers into a port table and a node table,
which also consumes memory.</cell>
</row>
<row>
<cell>Reference</cell>
<cell>On 32-bit architectures: 5 words for a reference from the
current local node + 7 words for a reference from another
node.<br></br>
On 64-bit architectures: 4 words for a reference from the current
local node + 6 words for a reference from another node.<br></br>
A reference refers into a node table, which also consumes
memory.</cell>
</row>
<row>
<cell>Fun</cell>
<cell>9..13 words + the size of environment.<br></br>
A fun refers into a fun table, which also consumes memory.</cell>
</row>
<row>
<cell>Ets table</cell>
<cell>Initially 768 words + the size of each element (6 words +
the size of Erlang data). The table grows when necessary.</cell>
</row>
<row>
<cell>Erlang process</cell>
<cell>338 words when spawned, including a heap of 233 words.</cell>
</row>
<tcaption>Memory Size of Different Data Types</tcaption>
</table>
</section>
<section>
<title>System Limits</title>
<p>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.</p>
<table>
<row>
<cell>Processes</cell>
<cell>The maximum number of simultaneously alive Erlang processes
is by default 32,768. This limit can be configured at startup.
For more information, see the
<seealso marker="erts:erl#max_processes"><c>+P</c></seealso>
command-line flag in the
<seealso marker="erts:erl"><c>erl(1)</c></seealso>
manual page in ERTS.</cell>
</row>
<row>
<cell>Known nodes</cell>
<cell>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
<seealso marker="#atoms">maximum number of atoms</seealso>
available for node names. All data concerning remote nodes,
except for the node name atom, are garbage-collected.</cell>
</row>
<row>
<cell>Connected nodes</cell>
<cell>The maximum number of simultaneously connected nodes is
limited by either the maximum number of simultaneously known
remote nodes,
<seealso marker="#ports">the maximum number of (Erlang) ports</seealso>
available, or
<seealso marker="#files_sockets">the maximum number of sockets</seealso>
available.</cell>
</row>
<row>
<cell>Characters in an atom</cell>
<cell>255.</cell>
</row>
<row>
<cell><marker id="atoms"></marker>Atoms</cell>
<cell>By default, the maximum number of atoms is 1,048,576. This
limit can be raised or lowered using the <c>+t</c> option.</cell>
</row>
<row>
<cell>Ets tables</cell>
<cell>Default is 1400. It can be changed with the environment
variable <c>ERL_MAX_ETS_TABLES</c>.</cell>
</row>
<row>
<cell>Elements in a tuple</cell>
<cell>The maximum number of elements in a tuple is 67,108,863
(26-bit unsigned integer). Clearly, other factors such as the
available memory can make it difficult to create a tuple of
that size.</cell>
</row>
<row>
<cell>Size of binary</cell>
<cell>In the 32-bit implementation of Erlang, 536,870,911
bytes is the largest binary that can be constructed or matched
using the bit syntax. In the 64-bit implementation, the maximum
size is 2,305,843,009,213,693,951 bytes. If the limit is
exceeded, bit syntax construction fails with a
<c>system_limit</c> exception, while any attempt to match a
binary that is too large fails. This limit is enforced starting
in R11B-4.<br></br>
In earlier Erlang/OTP releases, operations on too large
binaries in general either fail or give incorrect results. In
future releases, other operations that create binaries (such as
<c>list_to_binary/1</c>) will probably also enforce the same
limit.</cell>
</row>
<row>
<cell>Total amount of data allocated by an Erlang node</cell>
<cell>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.</cell>
</row>
<row>
<cell>Length of a node name</cell>
<cell>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.</cell>
</row>
<row>
<cell><marker id="ports"></marker>Open ports</cell>
<cell>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
<seealso marker="erts:erl#max_ports"><c>+Q</c></seealso>
command-line flag in the
<seealso marker="erts:erl"><c>erl(1)</c></seealso> manual page
in ERTS.</cell>
</row>
<row>
<cell><marker id="files_sockets"></marker>Open files and
sockets</cell>
<cell>The maximum number of simultaneously open files and
sockets depends on
<seealso marker="#ports">the maximum number of Erlang ports</seealso>
available, as well as on operating system-specific settings
and limits.</cell>
</row>
<row>
<cell>Number of arguments to a function or fun</cell>
<cell>255</cell>
</row>
<row>
<cell><marker id="unique_references"/>Unique References on a Runtime System Instance</cell>
<cell>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 <c>2⁶⁴ - 1</c> unique references. That is, the total
amount of unique references that can be produced on a runtime
system instance is <c>(NoSchedulers + 1) × (2⁶⁴ - 1)</c>.
<br/><br/>
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 unique enough.</cell>
</row>
<row>
<cell><marker id="unique_integers"/>Unique Integers on a Runtime System Instance</cell>
<cell>
There are two types of unique integers both created using the
<seealso marker="erts:erlang#unique_integer/1">erlang:unique_integer()</seealso>
BIF:
<br/><br/>
<em>1.</em> Unique integers created <em>with</em> the
<c>monotonic</c> modifier consist of a set of <c>2⁶⁴ - 1</c>
unique integers.
<br/><br/>
<em>2.</em> Unique integers created <em>without</em> the
<c>monotonic</c> modifier consist of a set of <c>2⁶⁴ - 1</c>
unique integers per scheduler thread and a set of <c>2⁶⁴ - 1</c>
unique integers shared by other threads. That is, the total
amount of unique integers without the <c>monotonic</c> modifier
is <c>(NoSchedulers + 1) × (2⁶⁴ - 1)</c>.
<br/><br/>
If a unique integer is created each nano second, unique integers
will at earliest be reused after more than 584 years. That is, for
the foreseeable future they are unique enough.
</cell>
</row>
<tcaption>System Limits</tcaption>
</table>
</section>
</chapter>
