This is regarding the statement WAIT FOR ASYNCHRONOUS TASKS and the corresponding part of the documentation:
If the result of log_exp is false and there is an asynchronous function call with callback routine, the program waits until a callback routine of a previous function (called asynchronously) has been executed and then checks the logical expression again:
Let's say I spawn 4 tasks, each reducing availability attribute by one, reaching 0. In the callback, they increase the availability attribute by one.
Now when I reach WAIT FOR ASYNCHRONOUS TASKS UNTIL availability > 0 UP TO 6000 SECONDS. the program waits until the counter is increased by a callback.
Question: When the logical expression is checked again, is it guaranteed that the order is
callback->check->callback->check?
Or could it be that availability is e.g. already 3, since it did
callback->callback->callback->check?
It works as per documentation.
WAIT -> CALLBACK -> CHECK , WAIT -> CALLBACK -> CHECK,
until wait condition is true or no more outstanding Callbacks are open.
It is important that the Callback form/method has finished before the check is performed as that routine is responsible for changing the variable/s in the WAIT UNTIL Condition.
An extract from the docu.
If the result of log_exp is false and there is an asynchronous
function call with callback routine, the program waits until a
callback routine of a previous function (called asynchronously) has
been executed and then checks the logical expression again:
If you are concerned about 2 callbacks occurring concurrently,
the callbacks are handled by the kernel sequentially.
There is no guarantee of order, just that the call backs are processed sequentially. Note the waiting program is only executed in 1 Work process at a time. From my tests, it is always the same Work process.
Related
I am trying to compare produer consumer problem implementation using cond variable and semaphores.
Implementation using cond variable:
acquire(m); // Acquire this monitor's lock.
while (!p) { // While the condition/predicate/assertion that we are waiting for is not true...
wait(m, cv); // Wait on this monitor's lock and condition variable.
}
// ... Critical section of code goes here ...
signal(cv2); -- OR -- notifyAll(cv2); // cv2 might be the same as cv or different.
release(m);
Implementation using semaphore:
produce:
P(emptyCount)
P(useQueue)
putItemIntoQueue(item)
V(useQueue)
V(fullCount)
why semaphore implementation is not using while loop to check the condition like in cond variable implementation.?
while (!p) { // While the condition/predicate/assertion that we are waiting for is not true...
wait(m, cv); // Wait on this monitor's lock and condition variable.
}
Why do you need a while loop while waiting for a condition variable
Grabbing a semaphore does use a tight loop internally, just like the cond version, but it yields execution back to the scheduler in each iteration to not waste resources like a busy loop would.
When the scheduler has executed some other process for a while, it yields execution back to your thread. If the semaphore is available now, it is grabbed; otherwise it yields back to the scheduler to let some other process run some more before retrying.
I was told that I could use Grand Central Dispatch to run n processes simultaneously, in an asynchronous fashion. The documentation said that if the processes were in a for loop, I could use the function dispatch_apply. But now it's saying
Note that dispatch_apply is synchronous, so all the applied blocks
will have completed by the time it returns.
Does this mean the blocks that are submitted to a queue using dispatch_apply are executed in order? If so, what is the point of using concurrency? Won't the slowdown be the same?
dispatch_apply is, as stated in the docs, synchronous. It runs a block on the specified queue in parallel (if possible) and waits until all the blocks return. If you want to run a block just once asynchronously, use dispatch_async, if you want to run a block multiple times in parallel without blocking your current queue, just call dispatch_apply within dispatch_async:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
dispatch_apply(10, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^(size_t size) {
NSLog(#"%lu", size);
});
});
The purpose of the synchronous dispatch_apply is to asynchronously dispatch the inner loop interations to available parallel processing resources. Thus, the overall loop performance may speed up.
Faster loop performance? Very possibly, Yes. (see caveat)
Blocks the thread calling dispatch_apply? Yes, just like loop blocks until completed.
For GCD, dispatch_apply is synchronous since dispatch_apply will not return until all the asynchronous, parallel tasks that dispatch_apply creates have completed.
However, each individual task enqueued by dispatch_apply can run as concurrent asynchronous tasks if the target queue is asynchronous.
For example in Swift:
let batchCount: Int = 10
let queue = dispatch_get_global_queue(QOS_CLASS_UTILITY, 0)
dispatch_apply(batchCount, queue) {
(i: Int) -> Void in
print(i, terminator: " ")
}
print("\ndispatch_apply QOS_CLASS_UTILITY queue completed")
yields unordered output like:
0 8 1 9 2 3 4 5 6 7
dispatch_apply QOS_CLASS_UTILITY queue completed
So, dispatch_apply synchronously blocks when called, but the "batch" of tasks generated by dispatch_apply can run concurrently, asynchronously, in parallel to each other.
Keep in mind the caveat that ...
the work performed during each iteration is distinct from the work
performed during all other iterations, and the order in which each
successive loop finished is unimportant
Also, note that use of a serial queue for the inner loop tasks will not have any performance gain.
Although using a serial queue is permissible and does the right thing
for your code, using such a queue has no real performance advantages
over leaving the loop in place.
You can get performance speed up by using gcl_create_dispatch_queue() with dispatch_apply()
For example:
#import Foundation;
#import OpenCL; // gcl_create_dispatch_queue()
int main() {
dispatch_queue_t queue = gcl_create_dispatch_queue(CL_DEVICE_TYPE_ALL, NULL);
dispatch_apply(10, queue, ^(size_t t) {
// some code here
});
}
More info:
OpenCL Programming Guide for Mac
There is one Producer and n consumers.
producer us assigning n jobs to n consumers and calling pthread_cond_wait() n times to wait for the assigned job to be completed by consumers.
Each Consumer after consuming job calls pthread_cond_signal() to notify the producer.
My question is "Will n calls to pthread_cond_signal() by consumer makes the producer to come out of pthread_cond_wait() n times? Or is there any case where multiple signals be merged into single signal so that pthread_cond_wait() comes out less than n times?
If the producer isn't actually waiting inside a call to pthread_cond_wait() when a consumer thread calls pthread_cond_signal(), then that signal will get 'lost' (ie., if the producer thread later rolls into the pthread_cond_wait(), it will block until another signal is sent).
That is why condition variables must be used in conjunction with some other "boolean predicate" that is checked while holding the mutex used with the condition variable. That predicate is the actual final word on whether or not the thread deciding whether or not to wait should wait. Another reason that the predicate is the final word is that a thread blocked in pthread_cond_wait() can be spuriously awakened.
From the POSIX docs on pthread_cond_wait():
When using condition variables there is always a Boolean predicate
involving shared variables associated with each condition wait that is
true if the thread should proceed. Spurious wakeups from the
pthread_cond_timedwait() or pthread_cond_wait() functions may occur.
Since the return from pthread_cond_timedwait() or pthread_cond_wait()
does not imply anything about the value of this predicate, the
predicate should be re-evaluated upon such return.
I am going through process synchronization, and facing difficulty in understanding semaphore. So here is my doubt:
the source says that
" Semaphore S is an integer variable that is accessed through standard atomic operations i.e. wait() and signal().
It also provided basic definition of wait()
wait(Semaphore S)
{
while S<=0
; //no operation
S--;
}
Definition of signal()
signal(S)
{
S++;
}
Let the initial value of a semaphore be 1, and say there are two concurrent processes P0 and P1 which are not supposed to perform operations of their critical section simultaneously.
Now say P0 is in its critical section, so the Semaphore S must have value 0, now say P1 wants to enter its critical section so it executes wait(), and in wait() it continuously loops, now to exit from the loop the semaphore value must be incremented, but it may not be possible because according the source, wait() is an atomic operation and can't be interrupted and thus the process P0 can't call signal() in a single processor system.
I want to know, is the understanding i have so far is correct or not. and if correct then how come process P0 call signal() when process P1 is strucked in while loop?
I think the top-voted answer is inaccurate!
Operation wait() and signal() must be completely atomic; no two processes can execute wait() or signal() operation simultaneously because they are implemented in kernel and processes in kernel mode can not be preempted.
If several processes attempt a P(S) simultaneously, only one process will be allowed to proceed(non-preemptive kernel that is free of race condition).
for the above implementation to work preemption is necessary (preemptive kernel)
read about the atomicity of semaphore operations
http://personal.kent.edu/~rmuhamma/OpSystems/Myos/semaphore.htm
https://en.wikibooks.org/wiki/Operating_System_Design/Processes/Semaphores
I think it's an inaccuracy in your source. Atomic for the wait() operation means each iteration of it is atomic, meaning S-- is performed without interruption, but the whole operation is interruptible after each completion of S-- inside the while loop.
I don't think, keeping an infinite while loop inside the wait() operation is wise. I would go for Stallings' example;
void semWait(semaphore s){
s.count--;
if(s.count<0)
*place this process in s.queue and block this process
}
I think what the book means for the atomic operation is testing S<=0 to be true as well as S--. Just like testAndset() it mention before.
if both separate operations S<=0 and S-- are atomic but can be interrupt by other process, this method won't work.
imagine two process p0 and p1, if p0 want to enter the critical section and tested S<=0 to be true. and it was interrupted by p1 and tested S<=0 also be true. then both of the process will enter the critical section. And that's wrong.
the actual not atomic operation is inside the while loop, even if the while loop is empty, other process can still interrupt current one when S<=0 tested to be false, which enable other process can continue their work in critical section and release the lock.
however, I think the code from the book can not actually use in OS since I don't know how to make operations S<=0 to be true and S-- together atomic. more possible way to do that is put the S-- inside the while loop like SomeWittyUsername said.
When a task attempts to acquire a semaphore that is unavailable, the semaphore places the task onto a wait queue and puts the task to sleep.The processor is then free to execute other code.When the semaphore becomes available, one of the tasks on the wait queue is awakened so that it can then acquire the semaphore.
while S<=0
; //no operation This doesn't mean that the processor running this code. The process/task is blocked until it gets the semaphore.
i think ,
when process P1 is strucked in while loop it will be in the wait state.processor will switch over among the process p0 & p1 (context switching) so the priority goes to p0 and it call signal() and then s will be incremented by 1 and p0 exit from the section so process P1 can enter into critical section and can avoid the mutual exclusion
i have a function it will take 3 to 30 secs time for execution depends on some calculations.
i want to stop if my function call takes more than 5 secs.
how to do this in Objective C.
You have to execute command in a separate thread (with performSelectorInBackground or with NSThread, for example), wait for 5 seconds (again, with unix sleep or NSThread methods) and then (depending on what is being done in the execution thread):
set some field in a class to "terminate", and check this field often in a "long" function
cancel IO operation, if there is a block
cancel a thread (you can read about it here: how to stop nsthread)
You can use a timer and put a condition like if the timer exceeds 5 seconds, do nothing.