= Thread Affinity

image::docs/images/Thread-Affinity_line.png[width=20%]

== Version

[#image-maven]
[caption="", link=https://maven-badges.herokuapp.com/maven-central/net.openhft/affinity]
image::https://maven-badges.herokuapp.com/maven-central/net.openhft/affinity/badge.svg[]
image:https://javadoc.io/badge2/net.openhft/affinity/javadoc.svg[link="https://www.javadoc.io/doc/net.openhft/affinity/latest/index.html"]

== Overview
Lets you bind a thread to a given core, this can improve performance (this library works best on linux).

OpenHFT Java Thread Affinity library

See https://github.com/OpenHFT/Java-Thread-Affinity/tree/master/affinity/src/test/java[affinity/src/test/java] 
for working examples of how to use this library.

=== Changes

* V3.2.0 - Add support for text configuration
* V3.1.1 - Upgraded JNA dependency to 4.4.0
* V2.0.1 - Added getThreadId for the process if of the thread.

=== Dependencies

Java-Thread-Affinity will try to use https://github.com/java-native-access/jna[JNA]
to provide access to native thread-handling functions. JNA should be installed on
your system to get the most from this library.

=== JNA version

Java-Thread-Affinity currently depends on JNA version 4.4.0, which in turn
depends on a version of GLIBC >= 2.14. If your operating system is an old one,
with a version of GLIBC released before 2011, this library will not be able to 
invoke native functions.

To work around this problem, fork the repository, and override the `<version>` tag
for the artifacts `jna` and `jna-platform` in the project's `pom` file.

=== Installing JNA on Ubuntu

    sudo apt-get install libjna-java

=== Installing JNA on CentOS

    sudo yum install jna

=== How does CPU allocation work?
The library will read your `/proc/cpuinfo` if you have one or provide one and it will determine your CPU layout.  If you don't have one it will assume every CPU is on one CPU socket.

The library looks for isolated CPUs determined by looking at the CPUs you are not running on by default. 
i.e. if you have 16 CPUs but 8 of them are not available for general use (as determined by the affinity of the process on startup) it will start assigning to those CPUs.

Note: if you have more than one process using this library you need to specify which CPUs the process can use otherwise it will assign the same CPUs to both processes.
To control which CPUs a process can use, add -Daffinity.reserved={cpu-mask-in-hex} to the command line of the process.

Note: the CPU 0 is reserved for the Operating System, it has to run somewhere.

=== References

https://github.com/peter-lawrey/Java-Thread-Affinity/wiki/Getting-started

https://github.com/peter-lawrey/Java-Thread-Affinity/wiki/How-it-works

http://vanillajava.blogspot.co.uk/2013/07/micro-jitter-busy-waiting-and-binding.html

=== isolcpus

Java-Thread-Affinity requires that you first isolate some CPU's.

Once a CPU core is isolated, the Linux scheduler will not use the CPU core to run any user-space processes. The isolated CPUs will not participate in load balancing, and will not have tasks running on them unless explicitly assigned.

To isolate the 1st and 3rd CPU cores (CPU numbers start from 0) on your system, add the following to the kernel command line during boot:

isolcpus=1,3

== Using AffinityLock

=== Acquiring a CPU lock for a thread
You can acquire a lock for a CPU in the following way:

In Java 6
[source, java]
----
AffinityLock al = AffinityLock.acquireLock();
try {
     // do some work locked to a CPU.
} finally {
     al.release();
}
----

In Java 7 or 8
[source, java]
----
try (AffinityLock al = AffinityLock.acquireLock()) {
    // do some work while locked to a CPU.
}
----
You have further options such as

=== Acquiring a CORE lock for a thread
You can reserve a whole core.  If you have hyper-threading enabled, this will use one CPU and leave it's twin CPU unused.
[source, java]
----
try (AffinityLock al = AffinityLock.acquireCore()) {
    // do some work while locked to a CPU.
}
----
=== Controlling layout
You can chose a layout relative to an existing lock.
[source, java]
----
try (final AffinityLock al = AffinityLock.acquireLock()) {
    System.out.println("Main locked");
    Thread t = new Thread(new Runnable() {
        @Override
        public void run() {
            try (AffinityLock al2 = al.acquireLock(AffinityStrategies.SAME_SOCKET,
                                                   AffinityStrategies.ANY)) {
                 System.out.println("Thread-0 locked");
            }
        }
    });
    t.start();
}
----    
In this example, the library will prefer a free CPU on the same Socket as the first thread, otherwise it will pick any free CPU. 

=== Getting the thread id
You can get the current thread id using
[source, java]
----

int threadId = AffinitySupport.getThreadId();
----
=== Determining which CPU you are running on
You can get the current CPU being used by
[source, java]
----

int cpuId = AffinitySupport.getCpu();
----
=== Controlling the affinity more directly
The affinity of the process on start up is
[source, java]
----

long baseAffinity = AffinityLock.BASE_AFFINITY;
----   
The available CPU for reservation is
[source, java]
----
long reservedAffinity = AffinityLock.RESERVED_AFFINITY;
----    
If you want to get/set the affinity directly you can do
[source, java]
---- 
long currentAffinity = AffinitySupport.getAffinity();
AffinitySupport.setAffinity(1L << 5); // lock to CPU 5.
----   

=== Debugging affinity state

For a detailed of view of the current affinity state (as seen by the library),
execute the following script on Linux systems:

[source]
----
# change to the affinity lock-file directory (defaults to system property java.io.tmpdir)
$ cd /tmp

# dump affinity state
$ for i in "$(ls cpu-*)"; 
      do PID="$(cat $i | head -n1)"; TIMESTAMP="$(cat $i | tail -n1)"; 
      echo "pid $PID locked at $TIMESTAMP in $i"; taskset -cp $PID; 
      cat "/proc/$PID/cmdline"; echo; echo 
  done

  pid 14584 locked at 2017.10.30 at 10:33:24 GMT in cpu-3.lock
  pid 14584's current affinity list: 3
  /opt/jdk1.8.0_141/bin/java ...

----

== Support Material

https://groups.google.com/forum/?hl=en-GB#!forum/java-thread-affinity[Java Thread Affinity support group]

For an article on how much difference affinity can make and how to use it http://vanillajava.blogspot.com/2013/07/micro-jitter-busy-waiting-and-binding.html

== Questions and Answers

=== Question: How to lock a specific cpuId
I am currently working on a project related to deadlock detection in multithreaded programs in java. We are trying to run threads on different processors and thus came across your github posts regarding the same. https://github.com/peter-lawrey/Java-Thread-Affinity/wiki/Getting-started
Being a beginner, I have little knowledge and thus need your assistance. We need to know how to run threads on specified cpu number and then switch threads when one is waiting. 

=== Answer

[source, java]
----
// lock a cpuId
try (AffinityLock lock = AffinityLock.acquireLock(n)) {

}
----

where n is the cpu you want to run the thread on.

OR

[source,java]
----
// lock one of the last CPUs
try (AffinityLock lock = AffinityLock.acquireLockLastMinus(n)) {

}
----

=== Question: how to use a configuration file to set the cpuId

I have the cpuId in a configuration file, how can I set it using a string?

=== Answer: use one of the following

[source,java]
----
try (AffinityLock lock = AffinityLock.acquireLock("last")) {
    assertEquals(PROCESSORS - 1, Affinity.getCpu());
}
try (AffinityLock lock = AffinityLock.acquireLock("last-1")) {
    assertEquals(PROCESSORS - 2, Affinity.getCpu());
}
try (AffinityLock lock = AffinityLock.acquireLock("1")) {
    assertEquals(1, Affinity.getCpu());
}
try (AffinityLock lock = AffinityLock.acquireLock("any")) {
    assertTrue(lock.bound);
}
try (AffinityLock lock = AffinityLock.acquireLock("none")) {
    assertFalse(lock.bound);
}
try (AffinityLock lock = AffinityLock.acquireLock((String) null)) {
    assertFalse(lock.bound);
}
try (AffinityLock lock = AffinityLock.acquireLock("0")) { // prints a warning
    assertFalse(lock.bound);
}
----
