© andy wellings, 2004 thread priorities i although priorities can be given to java threads,...

© Andy Wellings, 2004

Thread Priorities I

Although priorities can be given to Java threads, they are only used as a guide to the underlying scheduler when allocating resources

An application, once running, can explicitly give up the processor resource by calling the yield method

yield places the thread to the back of the run queue for its priority level


Thread Priorities IIpackage java.lang;public class Thread extends Object implements Runnable { // constants public static final int MAX_PRIORITY = 10; public static final int MIN_PRIORITY = 1; public static final int NORM_PRIORITY = 5; // methods public final int getPriority(); public final void setPriority(int newPriority); public static void yield(); ...}


Warning From a real-time perspective, Java’s

scheduling and priority models are weak; in particular: no guarantee is given that the highest

priority runnable thread is always executing

equal priority threads may or may not be time sliced

where native threads are used, different Java priorities may be mapped to the same operating system priority


Delaying Threads: Clocks Java supports the notion of a wall clock java.lang.System.currentTimeMillis returns the

number of milliseconds since 1/1/1970 GMT and is used by used by java.util.Date (see also java.util.Calendar)

However, a thread can only be delayed from executing by calling the sleep methods in the Thread class

sleep provides a relative delay (sleep from now for X milliseconds, y nano seconds), rather than sleep until 15th December 2003


Delaying a Threadpublic class Thread extends Object implements Runnable{ ... public static void sleep(long ms) throws InterruptedException; public static void sleep(long ms, int nanoseconds) throws InterruptedException; ...}


Sleep Granularity

Time specified by program

Granularity difference between clock and sleep time

Interrupts disabled

Thread runnable here but not executing

Thread executing

Time

Local drift


Drift The time over-run associated with both relative and

absolute delays is called the local drift and it it cannot be eliminated

It is possible, with absolute delays, to eliminate the cumulative drift that could arise if local drifts were allowed to superimpose

while(true) { // do action every 1 second sleep(1000)}


Absolute Delays I Consider an embedded system where the software controller

needs to invoke two actions The causes the environment to prepare for the second action The second action must occur a specified period (say 10

seconds) after the first action has been initiated Simply sleeping for 10 seconds after a call to the first action

will not achieve the desired effect for two reasons The first action may take some time to execute. If it took 1

second then a sleep of 10 would be a total delay of 11 seconds The thread could be pre-empted after the first action and not

execute again for several seconds This makes it extremely difficult to determine how long the

relative delay should be


Absolute Delays II{ static long start; static void action1() {...}; static void action2() {...}; try { start = System.currentTimeMillis(); action1(); Thread.sleep( 10000 - (System.currentTimeMillis() - start)); } catch (InterruptedException ie) {...}; action2();}

What is wrong with this approach?


Timeout on Waiting I In many situations, a thread can wait for an

arbitrary long period time within synchronized code for an associated notify (or notifyAll) call

There are occasions when the absence of the call, within a specified period of time, requires that the thread take some alternative action

Java provides two methods for this situation both of which allows the wait method call to timeout

In one case, the timeout is expressed in milliseconds; in the other case, milliseconds and nanoseconds can be specified


Timeout on Waiting II There are two important points to note about this

timeout facility

1. As with sleep, the timeout is a relative time and not an absolute time

2. It is not possible to know for certain if the thread has been woken by the timeout expiring or by a notify

There is no return value from the wait method and no timeout exception is thrown


Timeouts on Waitingpublic class TimeoutException extends Exception{};

public class TimedWait{ public static void wait(Object lock, long millis) throws InterruptedException, TimeoutException { // assumes the lock is held by the caller long start = System.currentTimeMillis(); lock.wait(millis); if(System.currentTimeMillis() >= start + millis) throw new TimeoutException(); }} What is wrong with this approach?


Thread Groups I Thread groups allow collections of threads to be

grouped together and manipulated as a group rather than as individuals

They also provide a means of restricting who does what to which thread

Every thread in Java is a member of a thread group There is a default group associated with the main

program, and hence unless otherwise specified, all created threads are placed in this group


Thread Groups IIpublic class ThreadGroup { public ThreadGroup(String name); // Creates a new thread group. public ThreadGroup(ThreadGroup parent, String name); // Creates a new group with the // specified parent.

. . .

public final void interrupt(); // Interrupt all threads in the group. public void uncaughtException(Thread t, Throwable e); // Called if a thread in the group // terminates due to an uncaught exception.}


Thread Groups III Hierarchies of thread groups to be created

Thread groups seem to have fallen from favour in recent years

The deprecation of many of its methods means that there is little use for it

However, the interrupt mechanisms is a useful way of interacting with a group of threads

Also, the uncaughtException method is the only hook that Java 1.4 provides for recovering when a thread terminates unexpectedly


Processes Threads execute within the same virtual address

space and, therefore, have access to shared memory. The Java languages acknowledges that the Java

program might not be the only activity on the hosting computer and that it will executing under control of an operating system

Java, therefore, allows the programmer to create and interact with other processes under that host system

Java defines two classes to aid this interaction: java.lang.Process java.lang.Runtime

(look them up on the web and in the book)


Strengths of the Java Concurrency Model

The main strength is that it is simple and it is supported directly by the language

This enables many of the errors that potentially occur with attempting to use an operating system interface for concurrency do not exists in Java

The language syntax and strong type checking gives some protection

E.g., it is not possible to forget to end a synchronized block

Portability of programs is enhanced because the concurrency model that the programmer uses is the same irrespective of on which OS the program finally executes


Weaknesses I Lack of support for condition variable Poor support for absolute time and time-outs on

waiting No preference given to threads continuing after a

notify over threads waiting to gain access to the monitor lock for the first time

Poor support for priorities

Note Java 1.5 concurrency utilities will provide some help here


Weaknesses II Synchronized code should be kept as short as possible Nested monitor calls

should be avoided because the outer lock is not released when the inner monitor waits (to release the lock causes other problems)

can lead to deadlock occurring It is not always obvious when a nested monitor call is

being made: methods in a class not labelled as synchronized can still contain a

synchronized statement; methods in a class not labelled as synchronized can be overridden

with a synchronized method; method calls which start off as being unsynchronized may be used with a synchronized subclass

methods called via interfaces cannot be labelled as synchronized


Bloch’s Thread Safety Levels I

Immutable Objects are constant and cannot be changed

Thread-safe Objects are mutable but they can be used safely in a

concurrent environment as the methods are synchronized Conditionally thread-safe

Objects either have methods which are thread-safe, or have methods which are called in sequence with the lock held by the caller


Bloch’s Thread Safety Levels II

Thread compatible Instances of the class provide no synchronization However, instances of the class can be safely used in a

concurrent environment, if the caller provides the synchronization by surrounding each method (or sequence of method calls) with the appropriate lock

Thread-hostile Instances of the class should not be used in a concurrent

environment even if the caller provides external synchronization

Typically a thread hostile class is accessing static data or the external environment


Summary I Threads can have priorities but support is weak Threads can delay themselves by using the sleep

methods which only supports relative time periods (intervals); it is not possible to accurately sleep until an absolute time

Time-outs of waiting for events is supported via the wait methods but it is not easy to determine whether the timeout has expired or the event has occurred

Threads can be grouped together via the ThreadGroup class Hierarchies of groups can be formed and it is possible to

interrupt the whole group


Summary II Interaction with processes outside the virtual

machine via the Processes and RunTime classes The Java model has both strengths and weaknesses Bloch’s has defined thread safety levels

© andy wellings, 2004 thread priorities i although priorities can be given to java threads,...

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