Deadlock
• 4 necessary conditions
–
–
–
–
Mutual exclusion
Hold and wait
No preemption
Circular wait
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Figure 6.7 Deadlock example. Threads T1 and T2 hold
locks L1 and L2, respectively, and each thread attempts
to acquire the other lock, which cannot be granted.
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Preventing Deadlock
• Two major categories for dealing with deadlock
– Prevent deadlock from happening
– Detect deadlocks and then break the deadlock
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Lock Hierarchies – preventing deadlock
• Each lock is assigned a position in an ordering.
Must acquire the locks in order
• Assumes a priori knowledge of all the locks
needed.
• If need a lower lock after acquiring a higher
lock, could release the higher locks, acquire the
lower locks and then reacquire the higher locks.
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Monitors
• Abstract concept that could be implemented in
a language.
• Only allows one thread access to the data.
• Has one single entry point for several methods
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Figure 6.8 Monitors provide an abstraction of synchronization
in which only one thread can access the monitor’s data at any
time. Other threads are blocked either waiting to enter the
monitor or waiting on events inside the monitor.
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Figure 6.12 Multiple readers, single
writer support routines.
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Figure 6.13 Multiple readers, single-writer
support routines based on a single-condition
variable, but subject to spurious wake-ups.
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Thread Scheduling
• Two ways to have a thread
– Map multiple threads to a single OS aware thread
• Unbound threads
• Create and destroy overhead low
• If OS blocks this thread, all the mapped threads are
blocked also
– Map each thread to its own OS aware thread
• Bound threads
• Better performance expected
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Code Spec 6.18 POSIX Thread routine
for setting thread scheduling attributes.
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Figure 6.14 A 2D relaxation replaces—on
each iteration—all interior values by the
average of their four nearest neighbors.
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Figure 6.15
False Sharing – when a processor has more
from the shared memory in its cache than
is “allocated” to that processor
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Figure 6.16 2D Sucessive over-relaxation
program written using POSIX Threads.
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Figure 6.16 2D Sucessive over-relaxation
program written using POSIX Threads. (cont.)
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Figure 6.16 2D Sucessive over-relaxation
program written using POSIX Threads. (cont.)
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Figure 6.16 2D Sucessive over-relaxation
program written using POSIX Threads. (cont.)
0
1
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Figure 6.17 It’s often profitable to do useful
work while waiting for some long-latency
operation to complete.
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Figure 6.18 A split-phase barrier allows a thread
to do useful work while waiting for the other
threads to arrive at the barrier.
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Figure 6.19 A 1D over-relaxation replaces—on each
iteration—all interior values by the average of their
two nearest neighbors.
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Figure 6.20 Program for 1D successive
over-relaxation using a single-phase
barrier.
+1
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Figure 6.21 Program for 1D successive overrelaxation using a split-phase barrier.
+1
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Figure 6.21 Program for 1D successive overrelaxation using a split-phase barrier. (cont.)
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Code Spec 6.19 Java’s Thread class
methods.
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Figure 6.27 Count 3s solution in Java.
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Figure 6.27 Count 3s solution in Java.
(cont.)
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Figure 6.27 Count 3s solution in Java.
(cont.)
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Code Spec 6.20 Examples of Java’s
atomic objects.
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Figure 6.28 OpenMP
schedule (static)
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Code Spec 6.21 OpenMP parallel for
statement.
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Code Spec 6.22 OpenMP reduce
operation. The <op> choice comes from
the accompanying table.
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Code Spec 6.23 Atomic specification in
OpenMP.
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