slides8.ppt

Slides for Parallel Programming Techniques & Applications Using Networked Workstations & Parallel Computers 2nd ed., by B. Wilkinson & M. Allen, @ 2004 Pearson Education Inc. All rights reserved.
1
Programming with Shared Memory
Chapter 8

2
Shared memory multiprocessor
system
Any memory location can be accessible by any of the
processors.
A single address space exists, meaning that each memory
location is given a unique address within a single range of
addresses.
Generally, shared memory programming more convenient
although it does require access to shared data to be
controlled by the programmer (using critical sections etc.)

3
using a single bus

4
using a crossbar switch

5
Alternatives for Programming Shared
Memory Multiprocessors:
• Using heavy weight processes.
• Using threads. Example Pthreads
• Using a completely new programming language for
parallel programming - not popular. Example Ada.
• Using library routines with an existing sequential
programming language.
• Modifying the syntax of an existing sequential
programming language to create a parallel programing
language. Example UPC
• Using an existing sequential programming language
supplemented with compiler directives for specifying
parallelism. Example OpenMP

6
Using Heavyweight Processes
Operating systems often based upon notion of a process.
Processor time shares between processes, switching from
one process to another. Might occur at regular intervals or
when an active process becomes delayed.
Offers opportunity to deschedule processes blocked from
proceeding for some reason, e.g. waiting for an I/O operation
to complete.
Concept could be used for parallel programming. Not much
used
because of overhead but fork/join concepts used elsewhere.

7
FORK-JOIN construct

8
UNIX System Calls

9
UNIX System Calls
SPMD model with different code for master process and
forked slave process.

10
Differences between a process and threads

11
Pthreads
IEEE Portable Operating System Interface, POSIX, sec.
1003.1 standard

12
Detached Threads
It may be that thread are not bothered when a
thread it creates terminates and then a join not
needed.
Threads not joined are called detached threads.
When detached threads terminate, they are
destroyed and their resource released.

13
Pthreads Detached Threads

14
Statement Execution Order
Single processor: Processes/threads typically executed until
blocked.
Multiprocessor: Instructions of processes/threads interleaved in
time.
Example
Process 1 Process 2
Instruction 1.1 Instruction 2.1
Several possible orderings, including
Instruction 1.1
Instruction 1.2
Instruction 2.1
Instruction 1.3
Instruction 2.2
Instruction 2.3
assuming instructions cannot be divided into smaller steps.

15
If two processes were to print messages, for
example, the messages could appear in different
orders depending upon the scheduling of
processes calling the print routine.
Worse, the individual characters of each
message could be interleaved if the machine
instructions of instances of the print routine could
be interleaved.

16
Compiler/Processor Optimizations
Compiler and processor reorder instructions for optimization.
Example
The statements
a = b + 5;
x = y + 4;
could be compiled to execute in reverse order:
x = y + 4;
a = b + 5;
and still be logically correct.
May be advantageous to delay statement a = b + 5 because a
previous instruction currently being executed in processor needs
more time to produce the value for b. Very common for processors
to execute machines instructions out of order for increased speed .

17
Thread-Safe Routines
Thread safe if they can be called from multiple threads
simultaneously and always produce correct results.
Standard I/O thread safe (prints messages without
interleaving the characters).
System routines that return time may not be thread safe.
Routines that access shared data may require special care to
be made thread safe.

18
Accessing Shared Data
Accessing shared data needs careful control.
Consider two processes each of which is to add one to a
shared data item, x. Necessary for the contents of the
location x to be read, x + 1 computed, and the result written
back to the location:

19
Conflict in accessing shared variable

20
Critical Section
A mechanism for ensuring that only one process
accesses a particular resource at a time is to establish
sections of code involving the resource as so-called
critical sections and arrange that only one such critical
section is executed at a time
This mechanism is known as mutual exclusion.
This concept also appears in an operating systems.

21
Locks
Simplest mechanism for ensuring mutual exclusion of critical
sections.
A lock is a 1-bit variable that is a 1 to indicate that a process
has entered the critical section and a 0 to indicate that no
process is in the critical section.
Operates much like that of a door lock:
A process coming to the “door” of a critical section and
finding it open may enter the critical section, locking the door
behind it to prevent other processes from entering. Once the
process has finished the critical section, it unlocks the door
and leaves.

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Control of critical sections through
busy waiting

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Pthread Lock Routines
Locks are implemented in Pthreads with mutually exclusive
lock variables, or “mutex” variables:
.
pthread_mutex_lock(&mutex1);
critical section
pthread_mutex_unlock(&mutex1);
.
If a thread reaches a mutex lock and finds it locked, it will
wait for the lock to open. If more than one thread is waiting
for the lock to open when it opens, the system will select one
thread to be allowed to proceed. Only the thread that locks a
mutex can unlock it.

24
Deadlock
Can occur with two processes when one requires a resource
held by the other, and this process requires a resource held by
the first process.

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Deadlock (deadly embrace)
Deadlock can also occur in a circular fashion with several
processes having a resource wanted by another.

26
Pthreads
Offers one routine that can test whether a lock is actually
closed without blocking the thread:
pthread_mutex_trylock()
Will lock an unlocked mutex and return 0 or will return with
EBUSY if the mutex is already locked – might find a use in
overcoming deadlock.

27
Semaphores
A positive integer (including zero) operated upon by two
operations:
P operation on semaphore s
Waits until s is greater than zero and then decrements s by
one and allows the process to continue.
V operation on semaphore s
Increments s by one and releases one of the waiting
processes (if any).

28
P and V operations are performed indivisibly.
Mechanism for activating waiting processes is
also implicit in P and V operations. Though exact
algorithm not specified, algorithm expected to be
fair. Processes delayed by P(s) are kept in
abeyance until released by a V(s) on the same
semaphore.
Devised by Dijkstra in 1968. Letter P is from the
Dutch word passeren, meaning “to pass,” and
letter V is from the Dutch word vrijgeven, meaning
“to release.”)

29
Mutual exclusion of critical sections can be achieved with one
semaphore having the value 0 or 1 (a binary semaphore), which
acts as a lock variable, but the P and V operations include a process
scheduling mechanism:
Process 1 Process 2 Process 3
Noncritical section Noncritical section Noncritical section
. . .
. . .
. . .
P(s) P(s) P(s)
Critical section Critical section Critical section
V(s) V(s) V(s)
. . .
. . .
. . .
Noncritical section Noncritical section Noncritical section

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General semaphore
(or counting semaphore)
Can take on positive values other than zero and one.
Provide, for example, a means of recording the number of
“resource units” available or used and can be used to solve
producer/ consumer problems. - more on that in operating
system courses.
Semaphore routines exist for UNIX processes.
Not exist in Pthreads as such, though they can be written
Do exist in real-time extension to Pthreads.

31
Monitor
Suite of procedures that provides only way to access
shared resource. Only one process can use a monitor
procedure at any instant.
Could be implemented using a semaphore or lock to
protect entry, i.e.,
monitor_proc1()
{
lock(x);
.
monitor body
.
unlock(x);
return;
}

32
Condition Variables
Often, a critical section is to be executed if a specific global
condition exists; for example, if a certain value of a variable
has been reached.
With locks, the global variable would need to be examined at
frequent intervals (“polled”) within a critical section.
Very time-consuming and unproductive exercise.
Can be overcome by introducing so-called condition
variables.

33
Pthread Condition Variables
Pthreads arrangement for signal and wait:
Signals not remembered - threads must already be waiting for
a signal to receive it.

34
Language Constructs for Parallelism
Shared Data
Shared memory variables might be declared as shared with,
say,
shared int x;

35
par Construct
For specifying concurrent statements:
par {
S1;
S2;
.
.
Sn;
}

36
forall Construct
To start multiple similar processes together:
forall (i = 0; i < n; i++) {
S1;
S2;
.
.
Sm;
}
which generates n processes each consisting of the
statements forming the body of the for loop, S1, S2, …,
Sm. Each process uses a different value of i.

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Example
forall (i = 0; i < 5; i++)
a[i] = 0;
clears a[0], a[1], a[2], a[3], and a[4] to zero concurrently.

38
Dependency Analysis
To identify which processes could be executed together.
Example
Can see immediately in the code
forall (i = 0; i < 5; i++)
a[i] = 0;
that every instance of the body is independent of other
instances and all instances can be executed simultaneously.
However, it may not be that obvious. Need algorithmic way
of recognizing dependencies, for a parallelizing compiler.

39

40

41
OpenMP
An accepted standard developed in the late 1990s by a
group of industry specialists.
Consists of a small set of compiler directives, augmented
with a small set of library routines and environment variables
using the base language Fortran and C/C++.
The compiler directives can specify such things as the par
and forall operations described previously.
Several OpenMP compilers available.

42
For C/C++, the OpenMP directives are contained in #pragma
statements. The OpenMP #pragma statements have the
format:
#pragma omp directive_name ...
where omp is an OpenMP keyword.
May be additional parameters (clauses) after the directive
name for different options.
Some directives require code to specified in a structured
block (a statement or statements) that follows the directive
and then the directive and structured block form a “construct”.

43
OpenMP uses “fork-join” model but thread-based.
Initially, a single thread is executed by a master thread.
Parallel regions (sections of code) can be executed by
multiple threads (a team of threads).
parallel directive creates a team of threads with a specified
block of code executed by the multiple threads in parallel.
The exact number of threads in the team determined by one
of several ways.
Other directives used within a parallel construct to specify
parallel for loops and different blocks of code for threads.

44
Parallel Directive
#pragma omp parallel
structured_block
creates multiple threads, each one executing the specified
structured_block, either a single statement or a compound
statement created with { ...} with a single entry point and a
single exit point.
There is an implicit barrier at the end of the construct.
The directive corresponds to forall construct.

45

46
Number of threads in a team
Established by either:
1. num_threads clause after the parallel directive, or
2. omp_set_num_threads() library routine being previously
called,
or
3. the environment variable OMP_NUM_THREADS is defined
in the order given or is system dependent if none of the
above.
Number of threads available can also be altered automatically
to achieve best use of system resources by a “dynamic
adjustment” mechanism.

47
Work-Sharing
Three constructs in this classification:
sections
for
single
In all cases, there is an implicit barrier at the end of the
construct unless a nowait clause is included.
Note that these constructs do not start a new team of threads.
That done by an enclosing parallel construct.

48
Sections
The construct
#pragma omp sections
{
#pragma omp section
structured_block
#pragma omp section
structured_block
.
.
.
}
cause the structured blocks to be shared among threads in team.
#pragma omp sections precedes the set of structured blocks.
#pragma omp section prefixes each structured block.
The first section directive is optional.

49
For Loop
#pragma omp for
for_loop
causes the for loop to be divided into parts and parts shared
among threads in the team. The for loop must be of a simple
form.
Way that for loop divided can be specified by an additional
“schedule” clause. Example: the clause schedule (static,
chunk_size) cause the for loop be divided into sizes specified
by chunk_size and allocated to threads in a round robin
fashion.

50
Single
The directive
#pragma omp single
structured block
cause the structured block to be executed by one thread only.

51
Combined Parallel Work-sharing
Constructs
If a parallel directive is followed by a single for directive, it
can be combined into:
#pragma omp parallel for
for_loop
with similar effects, i.e. it has the effect of each thread
executing the same for loop.

52

53
Master Directive
The master directive:
#pragma omp master
structured_block
causes the master thread to execute the structured block.
Different to those in the work sharing group in that there is
no implied barrier at the end of the construct (nor the
beginning). Other threads encountering this directive will
ignore it and the associated structured block, and will move
on.

54
Synchronization Constructs
Critical
The critical directive will only allow one thread execute the
associated structured block. When one or more threads
reach the
critical directive:
#pragma omp critical name
structured_block
they will wait until no other thread is executing the same
critical section (one with the same name), and then one
thread will proceed to execute the structured block. name is
optional. All critical sections with no name map to one
undefined name.

55
Barrier
When a thread reaches the barrier
#pragma omp barrier
it waits until all threads have reached the barrier and then they
all proceed together.
There are restrictions on the placement of barrier directive in a
program. In particular, all threads must be able to reach the
barrier.

56
Atomic
The atomic directive
#pragma omp atomic
expression_statement
implements a critical section efficiently when the critical
section simply updates a variable (adds one, subtracts one,
or does some other simple arithmetic operation as defined
by expression_statement).

57
Flush
A synchronization point which causes thread to have a
“consistent” view of certain or all shared variables in memory. All
current read and write operations on the variables allowed to
complete and values written back to memory but any memory
operations in the code after flush are not started, thereby creating
a “memory fence”. Format:
#pragma omp flush (variable_list)
Only applied to thread executing flush, not to all threads in the
team.
Flush occurs automatically at the entry and exit of parallel and
critical directives (and combined parallel for and parallel sections
directives), and at the exit of for, sections, and single (if a no-wait
clause is not present).

58
Ordered
Used in conjunction with for and parallel for directives to
cause an iteration to be executed in the order that it
would have occurred if written as a sequential loop.
See Appendix C of textbook for further details.

59
Shared Memory Programming
Performance Issues

60
Shared Data in Systems with Caches
All modern computer systems have cache memory, high-
speed memory closely attached to each processor for holding
recently referenced data and code.
Cache coherence protocols
Update policy - copies of data in all caches are updated at the
time one copy is altered.
Invalidate policy - when one copy of data is altered, the same
data in any other cache is invalidated (by resetting a valid bit in
the cache). These copies are only updated when the
associated processor makes reference for it.

61
False Sharing
Different parts of block
required by different
processors but not
same bytes. If one
processor writes to
one part of the block,
copies of the complete
block in other caches
must be updated or
invalidated though the
actual data is not
shared.

62
Solution for False Sharing
Compiler to alter the layout of the data stored in the main
memory, separating data only altered by one processor into
different blocks.

63
Critical Sections Serializing Code
High performance programs should have as few as
possible critical sections as their use can serialize the
code.
Suppose, all processes happen to come to their critical
section together.
They will execute their critical sections one after the other.
In that situation, the execution time becomes almost that of
a single processor.

64
Illustration

65
Sequential Consistency
Formally defined by Lamport (1979):
A multiprocessor is sequentially consistent if the result
of any execution is the same as if the operations of all
the processors were executed in some sequential
order, and the operations of each individual
processors occur in this sequence in the order
specified by its program.
i.e. the overall effect of a parallel program is not
changed by any arbitrary interleaving of instruction
execution in time.

66
Sequential Consistency

67
Writing a parallel program for a system which is known to be sequentially
consistent enables us to reason about the result of the program.
Example
Process P1 Process 2
. .
data = new; .
flag = TRUE; .
. .
. while (flag != TRUE) { };
. data_copy = data;
. .
Expect data_copy to be set to new because we expect the
statement data = new to be executed before flag = TRUE and the
statement while (flag != TRUE) { } to be executed before data_copy
= data. Ensures that process 2 reads new data from another process
1. Process 2 will simple wait for the new data to be produced.

68
Program Order
Sequential consistency refers to “operations of each
individual processor .. occur in the order specified in its
program” or program order.
In previous figure, this order is that of the stored machine
instructions to be executed.

69
Compiler Optimizations
The order is not necessarily the same as the order of the
corresponding high level statements in the source program
as a compiler may reorder statements for improved
performance. In this case, the term program order will
depend upon context, either the order in the source program
or the order in the compiled machine instructions.

70
High Performance Processors
Modern processors usually reorder machine instructions
internally during execution for increased performance.
This does not alter a multiprocessor being sequential
consistency, if the processor only produces the final results in
program order (that is, retires values to registers in program
order which most processors do).
All multiprocessors will have the option of operating under
the sequential consistency model. However, it can severely
limit compiler optimizations and processor performance.

71
Example of Processor Re-ordering
. .
new = a * b; .
data = new; .
flag = TRUE; .
. .
. data_copy = data;
. .
Multiply machine instruction corresponding to new = a * b is
issued for execution. The next instruction corresponding to data =
new cannot be issued until the multiply has produced its result.
However the next statement, flag = TRUE, is completely
independent and a clever processor could start this operation
before the multiply has completed leading to the sequence:

72
. .
new = a * b; .
flag = TRUE; .
data = new; .
. .
. data_copy = data;
. .
Now the while statement might occur before new is assigned
to data, and the code would fail.
All multiprocessors have the option of operating under the
sequential consistency model, i.e. not reorder the instructions
and forcing the multiply instruction above to complete before
starting subsequent instruction which depend upon its result.

73
Relaxing Read/Write Orders
Processors may be able to relax the consistency in
terms of the order of reads and writes of one processor
with respect to those of another processor to obtain
higher performance, and instructions to enforce
consistency when needed.

74
Examples
Alpha processors
Memory barrier (MB) instruction - waits for all previously
issued memory accesses instructions to complete before
issuing any new
memory operations.
Write memory barrier (WMB) instruction - as MB but only on
memory write operations, i.e. waits for all previously issued
memory write accesses instructions to complete before
issuing any new memory write operations - which means
memory reads could be issued after a memory write
operation but overtake it and complete before the write
operation. (check)

75
SUN Sparc V9 processors
memory barrier (MEMBAR) instruction with four bits for
variations Write-to-read bit prevent any reads that follow
it being issued before all writes that precede it have
completed. Other: Write-to-write, read-to-read, read-to-
write.
IBM PowerPC processor
SYNC instruction - similar to Alpha MB instruction
(check differences)

76
Shared Memory Program Examples

77
Program
To sum the elements of an array, a[1000]:
int sum, a[1000];
sum = 0;
for (i = 0; i < 1000; i++)
sum = sum + a[i];

78
UNIX Processes
Calculation will be divided into two parts, one doing even i and
one doing odd i; i.e.,
Process 1 Process 2
sum1 = 0; sum2 = 0;
for (i = 0; i < 1000; i = i + 2) for (i = 1; i < 1000; i = i + 2)
sum1 = sum1 + a[i]; sum2 = sum2 + a[i];
Each process will add its result (sum1 or sum2) to an
accumulating result, sum :
sum = sum + sum1; sum = sum + sum2;
Sum will need to be shared and protected by a lock. Shared
data structure is created:

79
Shared memory locations for UNIX
program example

80

81

82

83

84

85

86
Pthreads Example
n threads created, each taking numbers from list to add to their
sums. When all numbers taken, threads can add their partial
results to a shared location sum.
The shared location global_index is used by each thread to
select the next element of a[].
After index is read, it is incremented in preparation for the next
element to be read.
The result location is sum, as before, and will also need to be
shared and access protected by a lock.

87
Shared memory locations for
Pthreads program example

88

89

90
Java Example

91

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