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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<!-- This document was generated using DocBuilder 3.3.3 -->
<HTML>
<HEAD>
<TITLE>Advanced</TITLE>
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<A NAME="7"><!-- Empty --></A>
<H2>7 Advanced</H2>
<A NAME="7.1"><!-- Empty --></A>
<H3>7.1 Memory</H3>
<P>A good start when programming efficient 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 current
beam emulator version 5.2 system (OTP release R9B). The unit of
measurement is memory words. There exists both a 32-bit and a 64-bit
implementation, and a word is therefore, respectively, 4 bytes and
8 bytes.
<P>
<CENTER>
<TABLE CELLSPACING=0 CELLPADDING=2 BORDER=1>
<CAPTION ALIGN=BOTTOM><EM>Memory size of different data types</EM></CAPTION>
<TR>
<TD ALIGN="CENTER" VALIGN="MIDDLE">
Data type
</TD>
<TD ALIGN="CENTER" VALIGN="MIDDLE">
Memory size
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Integer (-16#7FFFFFF < i <16#7FFFFFF)
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
1 word
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Integer (big numbers)
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
3..N words
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Atom
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
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. Note also, this table is
<STRONG>not</STRONG> garbage collected.
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Float
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
On 32-bit architectures: 4 words
<BR>
On 64-bit architectures: 3 words
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Binary
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
3..6 + data (can be shared)
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
List
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
1 words per element + the size of each element
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
String (is the same as a List of Integers)
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
2 words per character
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Tuple
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
2 words + the size of each element
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Pid
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
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.
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Port
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
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.
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Reference
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
On 32-bit architectures: 5 words for a reference from the current
local node, and 7 words for a reference from another node.
<BR>
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.
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Fun
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
9..13 words + size of environment. Note, a
fun refers into a code table which also consumes memory.
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Ets table
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Initially 768 words + the size of each element (6 words
+ size of Erlang data). The table will grow when necessary.
</TD>
</TR>
<TR>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
Erlang process
</TD>
<TD ALIGN="LEFT" VALIGN="MIDDLE">
327 words when spawned including a heap of 233 words.
</TD>
</TR>
</TABLE>
</CENTER>
<A NAME="7.2"><!-- Empty --></A>
<H3>7.2 System limits</H3>
<P>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.
The current implementation has a few limitations
that is good to know about since some of them can be of
great importance for the design of an application.
<P>
<DL>
<DT>
<STRONG>Processes</STRONG>
</DT>
<DD>
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
<A HREF="javascript:erlhref('../../', 'erts', 'erl.html#max_processes');">+P</A> in the
<A HREF="javascript:erlhref('../../', 'erts', 'erl.html');">erl(1)</A> documentation).
The maximum limit of 268435456 processes will at least on a 32-bit
architecture be impossible to reach due to memory shortage.
<BR>
</DD>
<DT>
<STRONG>Distributed nodes</STRONG>
</DT>
<DD>
<DL>
<DT>
Known nodes
</DT>
<DD>
A remote node Y has to be known to the 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
<A HREF="#atoms">maximum number of atoms</A>
available for node names. All data concerning remote nodes,
except for the node name atom, are garbage collected.
<BR>
</DD>
<DT>
Connected nodes
</DT>
<DD>
The maximum number of simultaneously connected nodes is limited by
either the maximum number of simultaneously known remote nodes,
<A HREF="#ports">the maximum number of (Erlang) ports</A>
available, or
<A HREF="#files_sockets">the maximum number of sockets</A>
available.
</DD>
</DL>
</DD>
<DT>
<STRONG>Characters in an atom</STRONG>
</DT>
<DD>
255
</DD>
<DT>
<STRONG>Atoms </STRONG>
</DT>
<DD>
<A NAME="atoms"><!-- Empty --></A>The maximum number of atoms is 1048576.
</DD>
<DT>
<STRONG>Ets-tables</STRONG>
</DT>
<DD>
default=1400, can be changed with the environment variable <CODE>ERL_MAX_ETS_TABLES</CODE>.
</DD>
<DT>
<STRONG>Elements in a tuple</STRONG>
</DT>
<DD>
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.
</DD>
<DT>
<STRONG>Length of binary</STRONG>
</DT>
<DD>
Unsigned
</DD>
<DT>
<STRONG>Total amount of data allocated by an Erlang node</STRONG>
</DT>
<DD>
The Erlang runtime system can use the whole 32 bit address space, but the operating
system often limits one single process to use less than that.
</DD>
<DT>
<STRONG>length of a node name</STRONG>
</DT>
<DD>
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.
</DD>
<DT>
<STRONG>Open ports</STRONG>
</DT>
<DD>
<A NAME="ports"><!-- Empty --></A>
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
<A HREF="javascript:erlhref('../../', 'kernel', 'erlang.html#ERL_MAX_PORTS');">ERL_MAX_PORTS</A>
in <A HREF="javascript:erlhref('../../', 'kernel', 'erlang.html');">erlang(3)</A>)
The maximum limit of 268435456 open ports will at least on a 32-bit
architecture be impossible to reach due to memory shortage.
<BR>
</DD>
<DT>
<STRONG>Open files, and sockets</STRONG>
</DT>
<DD>
<A NAME="files_sockets"><!-- Empty --></A>
The maximum number of simultaneously open files and sockets
depend on
<A HREF="#ports">the maximum number of Erlang ports</A>
available, and operating system specific settings and limits.
</DD>
<DT>
<STRONG>Number of arguments to a function or fun</STRONG>
</DT>
<DD>
256
</DD>
</DL>
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