Thread Priorities I

1
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

2
Thread Priorities II
package 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() ...
3
Warning

• From a real-time perspective, Javas 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

4
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

5
Delaying a Thread
public class Thread extends Object
implements Runnable ... public static
void sleep(long ms) throws
InterruptedException public static void
sleep(long ms, int
nanoseconds) throws InterruptedException
...
6
Sleep Granularity
Local drift
Granularity difference between clock and sleep
time
Time specified by program
Thread runnable here but not executing
Interrupts disabled
Time
7
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)
8
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

9
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?
10
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

11
Timeout on Waiting II

• There are two important points to note about this
  timeout facility
• As with sleep, the timeout is a relative time and
  not an absolute time
• 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

12
Timeouts on Waiting
public 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
() gt start millis) throw new
TimeoutException()
What is wrong with this approach?
13
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

14
Thread Groups II
public 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.
15
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

16
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)

17
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

18
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
19
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

20
Blochs 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

21
Blochs 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

22
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

23
Summary II

• Interaction with processes outside the virtual
  machine via the Processes and RunTime classes
• The Java model has both strengths and weaknesses
• Blochs has defined thread safety levels