path: root/erts/doc/src/counters.xml

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  <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 <seealso marker="#new/2"><c>new/2</c></seealso>.</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
        <seealso marker="#new/2"><c>new/2</c></seealso>.</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
	  <seealso marker="#add/3"><c>add</c></seealso> and <seealso
	  marker="#sub/3"><c>sub</c></seealso> 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 <seealso marker="#add/3"><c>add</c></seealso> and
	  <seealso marker="#sub/3"><c>sub</c></seealso> toward the same counters
	  are very frequent, while calls to <seealso marker="#get/2"><c>get</c>
	  </seealso> and <seealso marker="#put/3"><c>put</c></seealso> are much
	  less frequent. The lack of absolute read consistency must also be
	  acceptable.</p>
	  </item>
	</taglist>
      </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 <seealso
	  marker="#add/3"><c>add</c></seealso> and <seealso marker="#sub/3">
	  <c>sub</c></seealso>. The cost for a <c>put</c> with
	  <c>write_concurrency</c> is like a <seealso marker="#get/2"><c>get</c>
	  </seealso> 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>