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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>2018</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>counters</title>
</header>
<module since="OTP 21.2">counters</module>
<modulesummary>Counter Functions</modulesummary>
<description>
<p>This module provides a set of functions to do operations towards
shared mutable counter variables. The implementation does not utilize any
software level locking, which makes it very efficient for concurrent
access. The counters are organized into arrays with the following
semantics:</p>
<list type="bulleted">
<item>
<p>Counters are 64 bit signed integers.</p>
</item>
<item>
<p>Counters wrap around at overflow and underflow operations.</p>
</item>
<item><p>Counters are initialized to zero and can then only be written to
by adding or subtracting.</p>
</item>
<item>
<p>Write operations guarantee atomicity. No intermediate results can be
seen from a single write operation.</p>
</item>
<item>
<p>Two types of counter arrays can be created with options <c>atomics</c> or
<c>write_concurrency</c>. The <c>atomics</c> counters have good allround
performance with nice consistent semantics while
<c>write_concurrency</c> counters offers even better concurrent
write performance at the expense of some potential read
inconsistencies. See <seemfa marker="#new/2"><c>new/2</c></seemfa>.</p>
</item>
<item>
<p>Indexes into counter arrays are one-based. A counter array of
size N contains N counters with index from 1 to N.</p>
</item>
</list>
</description>
<datatypes>
<datatype>
<name name="counters_ref"/>
<desc><p>Identifies a counter array returned from
<seemfa marker="#new/2"><c>new/2</c></seemfa>.</p>
</desc>
</datatype>
</datatypes>
<funcs>
<func>
<name name="new" arity="2" since="OTP 21.2"/>
<fsummary>Create counter array</fsummary>
<desc>
<p>Create a new counter array of <c><anno>Size</anno></c> counters.</p>
<p>Argument <c><anno>Opts</anno></c> is a list of the following possible
options:</p>
<taglist>
<tag><c>atomics</c> (Default)</tag>
<item><p>Counters will be sequentially consistent. If write
operation A is done sequentially before write operation B, then a concurrent reader
may see none of them, only A, or both A and B. It cannot see only B.</p>
</item>
<tag><c>write_concurrency</c></tag>
<item><p>This is an optimization to achieve very efficient concurrent
<seemfa marker="#add/3"><c>add</c></seemfa> and <seemfa
marker="#sub/3"><c>sub</c></seemfa> operations at the expense of potential read
inconsistency and memory consumption per counter.</p>
<p>Read operations may see sequentially inconsistent results with
regard to concurrent write operations. Even if write operation A is done
sequentially before write operation B, a concurrent reader may see any
combination of A and B, including only B. A read operation is only
guaranteed to see all writes done sequentially before the read. No writes
are ever lost, but will eventually all be seen.</p>
<p>The typical use case for <c>write_concurrency</c> is when
concurrent calls to <seemfa marker="#add/3"><c>add</c></seemfa> and
<seemfa marker="#sub/3"><c>sub</c></seemfa> toward the same counters
are very frequent, while calls to <seemfa marker="#get/2"><c>get</c>
</seemfa> and <seemfa marker="#put/3"><c>put</c></seemfa> are much
less frequent. The lack of absolute read consistency must also be
acceptable.</p>
</item>
</taglist>
<p>Counters are not tied to the current process and are automatically
garbage collected when they are no longer referenced.</p>
</desc>
</func>
<func>
<name name="get" arity="2" since="OTP 21.2"/>
<fsummary>Read counter value</fsummary>
<desc>
<p>Read counter value.</p>
</desc>
</func>
<func>
<name name="add" arity="3" since="OTP 21.2"/>
<fsummary>Add to counter</fsummary>
<desc>
<p>Add <c><anno>Incr</anno></c> to counter at index
<c><anno>Ix</anno></c>.</p>
</desc>
</func>
<func>
<name name="sub" arity="3" since="OTP 21.2"/>
<fsummary>Subtract from counter</fsummary>
<desc>
<p>Subtract <c><anno>Decr</anno></c> from counter at index
<c><anno>Ix</anno></c>.</p>
</desc>
</func>
<func>
<name name="put" arity="3" since="OTP 21.2"/>
<fsummary>Set counter to value</fsummary>
<desc>
<p>Write <c><anno>Value</anno></c> to counter at index
<c><anno>Ix</anno></c>.</p>
<note>
<p>Despite its name, the <c>write_concurrency</c> optimization does not
improve <c>put</c>. A call to <c>put</c> is a relatively heavy
operation compared to the very lightweight and scalable <seemfa
marker="#add/3"><c>add</c></seemfa> and <seemfa marker="#sub/3">
<c>sub</c></seemfa>. The cost for a <c>put</c> with
<c>write_concurrency</c> is like a <seemfa marker="#get/2"><c>get</c>
</seemfa> plus a <c>put</c> without <c>write_concurrency</c>.</p>
</note>
</desc>
</func>
<func>
<name name="info" arity="1" since="OTP 21.2"/>
<fsummary>Get information about counter array.</fsummary>
<desc>
<p>Return information about a counter array in a map. The map
has the following keys (at least):</p>
<taglist>
<tag><c>size</c></tag>
<item><p>The number of counters in the array.</p></item>
<tag><c>memory</c></tag>
<item><p>Approximate memory consumption for the array in
bytes.</p></item>
</taglist>
</desc>
</func>
</funcs>
</erlref>
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