org.logicalcobwebs.concurrent
Class FJTaskRunnerGroup

java.lang.Object
  org.logicalcobwebs.concurrent.FJTaskRunnerGroup

All Implemented Interfaces:

Executor

public class FJTaskRunnerGroup
extends java.lang.Object
implements Executor

A stripped down analog of a ThreadGroup used for establishing and managing FJTaskRunner threads. ThreadRunnerGroups serve as the control boundary separating the general world of normal threads from the specialized world of FJTasks.

By intent, this class does not subclass java.lang.ThreadGroup, and does not support most methods found in ThreadGroups, since they would make no sense for FJTaskRunner threads. In fact, the class does not deal with ThreadGroups at all. If you want to restrict a FJTaskRunnerGroup to a particular ThreadGroup, you can create it from within that ThreadGroup.

The main contextual parameter for a FJTaskRunnerGroup is the group size, established in the constructor. Groups must be of a fixed size. There is no way to dynamically increase or decrease the number of threads in an existing group.

In general, the group size should be equal to the number of CPUs on the system. (Unfortunately, there is no portable means of automatically detecting the number of CPUs on a JVM, so there is no good way to automate defaults.) In principle, when FJTasks are used for computation-intensive tasks, having only as many threads as CPUs should minimize bookkeeping overhead and contention, and so maximize throughput. However, because FJTaskRunners lie atop Java threads, and in turn operating system thread support and scheduling policies, it is very possible that using more threads than CPUs will improve overall throughput even though it adds to overhead. This will always be so if FJTasks are I/O bound. So it may pay to experiment a bit when tuning on particular platforms. You can also use setRunPriorities to either increase or decrease the priorities of active threads, which may interact with group size choice.

In any case, overestimating group sizes never seriously degrades performance (at least within reasonable bounds). You can also use a value less than the number of CPUs in order to reserve processing for unrelated threads.

There are two general styles for using a FJTaskRunnerGroup. You can create one group per entire program execution, for example as a static singleton, and use it for all parallel tasks:

 class Tasks {
   static FJTaskRunnerGroup group;
   public void initialize(int groupsize) {
      group = new FJTaskRunnerGroup(groupSize);
   }
   // ...
 }
 

Alternatively, you can make new groups on the fly and use them only for particular task sets. This is more flexible,, and leads to more controllable and deterministic execution patterns, but it encounters greater overhead on startup. Also, to reclaim system resources, you should call FJTaskRunnerGroup.interruptAll when you are done using one-shot groups. Otherwise, because FJTaskRunners set Thread.isDaemon status, they will not normally be reclaimed until program termination.

The main supported methods are execute, which starts a task processed by FJTaskRunner threads, and invoke, which starts one and waits for completion. For example, you might extend the above FJTasks class to support a task-based computation, say, the Fib class from the FJTask documentation:

 class Tasks { // continued
   // ...
   static int fib(int n) {
     try {
       Fib f = new Fib(n);
       group.invoke(f);
       return f.getAnswer();
     }
     catch (InterruptedException ex) {
       throw new Error("Interrupted during computation");
     }
   }
 }
 

Method stats() can be used to monitor performance. Both FJTaskRunnerGroup and FJTaskRunner may be compiled with the compile-time constant COLLECT_STATS set to false. In this case, various simple counts reported in stats() are not collected. On platforms tested, this leads to such a tiny performance improvement that there is very little motivation to bother.

[ Introduction to this package. ]

See Also:

FJTaskRunnerGroup.InvokableFJTask, FJTaskRunner

Nested Class Summary

protected static class

FJTaskRunnerGroup.InvokableFJTask Wrap wait/notify mechanics around a task so that invoke() can wait it out

Field Summary

protected int	activeCount Number of threads that are not waiting for work
(package private) static boolean	COLLECT_STATS Compile-time constant.
(package private) static int	DEFAULT_SCAN_PRIORITY
(package private) int	entries Total number of executes or invokes
protected LinkedQueue	entryQueue Group-wide queue for tasks entered via execute()
(package private) long	initTime The time at which this ThreadRunnerGroup was constructed
(package private) static long	MAX_SLEEP_TIME The maximum time (in msecs) to sleep when a thread is idle, yet others are not, so may eventually generate work that the current thread can steal.
protected int	nstarted Number of threads that have been started.
(package private) static long	SCANS_PER_SLEEP The number of times to scan other threads for tasks before transitioning to a mode where scans are interleaved with sleeps (actually timed waits).
protected FJTaskRunner[]	threads The threads in this group

Constructor Summary

FJTaskRunnerGroup(int groupSize)
Create a FJTaskRunnerGroup with the indicated number of FJTaskRunner threads.

Method Summary

protected void	checkActive(FJTaskRunner t, long scans) Set active status of thread t to false, and then wait until: (a) there is a task in the entry queue, or (b) other threads are active, or (c) the current thread is interrupted.
void	execute(java.lang.Runnable r) Arrange for execution of the given task by placing it in a work queue.
void	executeTask(FJTask t) Specialized form of execute called only from within FJTasks
protected boolean	getActive(FJTaskRunner t) Return active status of t.
int	getActiveCount() Return the number of threads that are not idly waiting for work.
protected FJTaskRunner[]	getArray() Return the array of threads in this group.
protected void	initializeThreads() Create all FJTaskRunner threads in this group.
void	interruptAll() Try to shut down all FJTaskRunner threads in this group by interrupting them all.
void	invoke(java.lang.Runnable r) Start a task and wait it out.
protected FJTask	pollEntryQueue() Return a task from entry queue, or null if empty.
protected void	setActive(FJTaskRunner t) Set active status of thread t to true, and notify others that might be waiting for work.
protected void	setInactive(FJTaskRunner t) Set active status of thread t to false.
void	setRunPriorities(int pri) Set the priority to use while a FJTaskRunner is actively running tasks.
void	setScanPriorities(int pri) Set the priority to use while a FJTaskRunner is polling for new tasks to perform.
protected void	signalNewTask() Start or wake up any threads waiting for work
int	size() Return the number of FJTaskRunner threads in this group
void	stats() Prints various snapshot statistics to System.out.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

threads

protected final FJTaskRunner[] threads

The threads in this group

entryQueue

protected final LinkedQueue entryQueue

Group-wide queue for tasks entered via execute()

activeCount

protected int activeCount

Number of threads that are not waiting for work

nstarted

protected int nstarted

Number of threads that have been started. Used to avoid unecessary contention during startup of task sets.

COLLECT_STATS

static final boolean COLLECT_STATS

Compile-time constant. If true, various counts of runs, waits, etc., are maintained. These are NOT updated with synchronization, so statistics reports might not be accurate.

See Also:

Constant Field Values

initTime

long initTime

The time at which this ThreadRunnerGroup was constructed

entries

int entries

Total number of executes or invokes

DEFAULT_SCAN_PRIORITY

static final int DEFAULT_SCAN_PRIORITY

See Also:

Constant Field Values

SCANS_PER_SLEEP

static final long SCANS_PER_SLEEP

The number of times to scan other threads for tasks before transitioning to a mode where scans are interleaved with sleeps (actually timed waits). Upon transition, sleeps are for duration of scans / SCANS_PER_SLEEP milliseconds.

This is not treated as a user-tunable parameter because good values do not appear to vary much across JVMs or applications. Its main role is to help avoid some useless spinning and contention during task startup.

See Also:

Constant Field Values

MAX_SLEEP_TIME

static final long MAX_SLEEP_TIME

The maximum time (in msecs) to sleep when a thread is idle, yet others are not, so may eventually generate work that the current thread can steal. This value reflects the maximum time that a thread may sleep when it possibly should not, because there are other active threads that might generate work. In practice, designs in which some threads become stalled because others are running yet not generating tasks are not likely to work well in this framework anyway, so the exact value does not matter too much. However, keeping it in the sub-second range does help smooth out startup and shutdown effects.

See Also:

Constant Field Values

Constructor Detail

FJTaskRunnerGroup

public FJTaskRunnerGroup(int groupSize)

Create a FJTaskRunnerGroup with the indicated number of FJTaskRunner threads. Normally, the best size to use is the number of CPUs on the system.

The threads in a FJTaskRunnerGroup are created with their isDaemon status set, so do not normally need to be shut down manually upon program termination.

Method Detail

execute

public void execute(java.lang.Runnable r)
             throws java.lang.InterruptedException

Arrange for execution of the given task by placing it in a work queue. If the argument is not of type FJTask, it is embedded in a FJTask via FJTask.Wrap.

Specified by:

execute in interface Executor

Throws:

java.lang.InterruptedException - if current Thread is currently interrupted

executeTask

public void executeTask(FJTask t)

Specialized form of execute called only from within FJTasks

invoke

public void invoke(java.lang.Runnable r)
            throws java.lang.InterruptedException

Start a task and wait it out. Returns when the task completes.

Throws:

java.lang.InterruptedException - if current Thread is interrupted before completion of the task.

interruptAll

public void interruptAll()

Try to shut down all FJTaskRunner threads in this group by interrupting them all. This method is designed to be used during cleanup when it is somehow known that all threads are idle. FJTaskRunners only check for interruption when they are not otherwise processing a task (and its generated subtasks, if any), so if any threads are active, shutdown may take a while, and may lead to unpredictable task processing.

setScanPriorities

public void setScanPriorities(int pri)

Set the priority to use while a FJTaskRunner is polling for new tasks to perform. Default is currently Thread.MIN_PRIORITY+1. The value set may not go into effect immediately, but will be used at least the next time a thread scans for work.

setRunPriorities

public void setRunPriorities(int pri)

Set the priority to use while a FJTaskRunner is actively running tasks. Default is the priority that was in effect by the thread that constructed this FJTaskRunnerGroup. Setting this value while threads are running may momentarily result in them running at this priority even when idly waiting for work.

size

public int size()

Return the number of FJTaskRunner threads in this group

getActiveCount

public int getActiveCount()

Return the number of threads that are not idly waiting for work. Beware that even active threads might not be doing any useful work, but just spinning waiting for other dependent tasks. Also, since this is just a snapshot value, some tasks may be in the process of becoming idle.

stats

public void stats()

Prints various snapshot statistics to System.out.

• For each FJTaskRunner thread (labeled as Tn, for n from zero to group size - 1):
  • A star "*" is printed if the thread is currently active; that is, not sleeping while waiting for work. Because threads gradually enter sleep modes, an active thread may in fact be about to sleep (or wake up).
  • Q Cap The current capacity of its task queue.
  • Run The total number of tasks that have been run.
  • New The number of these tasks that were taken from either the entry queue or from other thread queues; that is, the number of tasks run that were not forked by the thread itself.
  • Scan The number of times other task queues or the entry queue were polled for tasks.
• Execute The total number of tasks entered (but not necessarily yet run) via execute or invoke.
• Time Time in seconds since construction of this FJTaskRunnerGroup.
• Rate The total number of tasks processed per second across all threads. This may be useful as a simple throughput indicator if all processed tasks take approximately the same time to run.

Cautions: Some statistics are updated and gathered without synchronization, so may not be accurate. However, reported counts may be considered as lower bounds of actual values. Some values may be zero if classes are compiled with COLLECT_STATS set to false. (FJTaskRunner and FJTaskRunnerGroup classes can be independently compiled with different values of COLLECT_STATS.) Also, the counts are maintained as ints so could overflow in exceptionally long-lived applications.

These statistics can be useful when tuning algorithms or diagnosing problems. For example:

• High numbers of scans may mean that there is insufficient parallelism to keep threads busy. However, high scan rates are expected if the number of Executes is also high or there is a lot of global synchronization in the application, and the system is not otherwise busy. Threads may scan for work hundreds of times upon startup, shutdown, and global synch points of task sets.
• Large imbalances in tasks run across different threads might just reflect contention with unrelated threads on a system (possibly including JVM threads such as GC), but may also indicate some systematic bias in how you generate tasks.
• Large task queue capacities may mean that too many tasks are being generated before they can be run. Capacities are reported rather than current numbers of tasks in queues because they are better indicators of the existence of these kinds of possibly-transient problems. Queue capacities are resized on demand from their initial value of 4096 elements, which is much more than sufficient for the kinds of applications that this framework is intended to best support.

getArray

protected FJTaskRunner[] getArray()

Return the array of threads in this group. Called only by FJTaskRunner.scan().

pollEntryQueue

protected FJTask pollEntryQueue()

Return a task from entry queue, or null if empty. Called only by FJTaskRunner.scan().

getActive

protected boolean getActive(FJTaskRunner t)

Return active status of t. Per-thread active status can only be accessed and modified via synchronized method here in the group class.

setActive

protected void setActive(FJTaskRunner t)

Set active status of thread t to true, and notify others that might be waiting for work.

setInactive

protected void setInactive(FJTaskRunner t)

Set active status of thread t to false.

checkActive

protected void checkActive(FJTaskRunner t,
                           long scans)

Set active status of thread t to false, and then wait until: (a) there is a task in the entry queue, or (b) other threads are active, or (c) the current thread is interrupted. Upon return, it is not certain that there will be work available. The thread must itself check.

The main underlying reason for these mechanics is that threads do not signal each other when they add elements to their queues. (This would add to task overhead, reduce locality. and increase contention.) So we must rely on a tamed form of polling. However, tasks inserted into the entry queue do result in signals, so tasks can wait on these if all of them are otherwise idle.

signalNewTask

protected void signalNewTask()

Start or wake up any threads waiting for work

initializeThreads

protected void initializeThreads()

Create all FJTaskRunner threads in this group.