Synchronization 1: Semaphores

This video presents the first of several synchronization tools: semaphores. Semaphores (specifically, counting semaphores) allow processes to send signals and request access to resources. When a resource is not available, the process waits until a signal is received. Based on content from the book Operating Systems: Internals and Design Principles, by William Stallings.

[@lokinasthenerd]

I have spent the past two weeks struggling to understand how semaphores work for a uni course I'm taking, and you have explained it more clearly in a little over 15 minutes than my course material and teacher could in those two weeks! Thank you so gd much!!

[@t3ntube357]

because of you I now unnderstand symaphore, thanks jacob

[@defneumut9197]

whoa! what an explanation, pure, simple... loved it

[@sirleo5103]

For the semsignal, shouldn't it be "If s >= 0" instead of "If s

[@Roogie22]

this channel with x2 speed is life

[@mostafaelgayar]

lol

[@Roogie22]

@@mostafaelgayar stalkerrrr

[@Marcos-pf6rx]

Thanks! Greetings from Argentina.

[@SuperSam4y0u]

Explained in a simple way 😊 the semaphore count could be the number of database connections we have in pool or other similar limited resource. I am just thinking the logic to decrement, check if negative and if yes then block the process must all be done as a atomic operation yes?

[@wolfram77]

OS has to do a lot of work if critical section does not complete. Is there a way for a process to signal that it is executing a critical section?

[@durumarthu]

Wait! Isn't sem_post() or what you call semSignal() require the semaphore value (s) to be >= 0 to be able to signal (wake) any waiting (sleeping) process or thread? If so, how could you have s=-3 and sill be able to wake (signal) processes that are in the waiting queue?

[@cb-sy5qu]

bounced on my boy's Semaphore to this for hours

[@anaibrahim4361]

before i finish your explanation i want to thank you for it
i finally arrived to understand all the bla bla bla in the univ and other channels about semaphores

[@s1mpl3us34]

We implements a counting semaphore where multiple threads can call sem_wait
and sem_post concurrently.
In addition to the integer value N associated with the Semaphore, the semaphore includes a
waiting list that keeps track of the waiting threads and a mutex lock that protects the shared data
inside the semaphore instance. If two thread gets access to the semi_wait and semi_post if we use lock in semwait and semi post is ti correct implementation?

[@souravdutta7120]

Really a very clear and apt explanation....
thank you

[@AndrewCheetah]

Do you use a watchdog to determine whether a process has timed out or not?

[@Cora-q1z]

very interesting and easy to understand. helped me in my project report on semaphores. Thumbs up

[@iwebappe7981]

why aren't you incrementing S each time the processus ends ?
it should be like this :
s=1
S=1-1=0
A executing
s=0+1=1
it is because of the time out ? i don't know it

[@christos94dim75]

semSignal increments s by 1, in this example "A" got interrupted BEFORE finishing its critical section, that means it didnt reach the sesSignal part. Thats the hole point of the semaphore, to not increment s until you are done with you "task", so nobody "messes" with your task , shared data etc.

[@Angeck]

Wait so A gets to go twice when C didn't even have one turn?

[@hamzabenki2864]

So well explained. Thank you very much !

[@tomerbendavid9974]

Excellent video!
Just watch it in 1.25x speed

[@user-qc9rq3vb2z]

would each respective semaphore have their own queue of blocked process/threads waiting to use it? Thanks in advance.

[@JacobSchrum]

Yes, each semaphore maintains its own queue

[@youneskyba9589]

Great explanation! Thanks.

[@yufengyang8182]

It's really helpful, thank you!

[@BelieveB23]

Great explanation sir! Well done. Thanks.

[@nirvana124]

MAY GOD BLESS YOU !! what a nice explanation. thank you so much _/\_

[@Dpfpv1]

can you have an interrupt increment s with semSignal without going through semWait?

[@vipudank]

Hi
I dont understand what you mean by ressource

[@ThefamousMrcroissant]

I'm far too late I'm sure, but: a resource is the object of the synchronization. Say you have a file and several writing threads trying to access it, then clearly you don't want to have them all access it at once. This is the "resource" he speaks of; threads will have to queue up to wait for it.

[@BenStoneking]

Great video!

[@rougejean3524]

thank you my nigga

[@Res977]

very helpful, thanks

[@בנירויטנברג]

הסבר מצויין !!!!! thenk you

[@Neuroszima]

wait. Wouldn't be more simple to just use bool instead of this????

[@JacobSchrum]

If you are suggesting replacing the entire concept of a semaphore with a single boolean variable, then you are overlooking the fact that updates to a boolean variable in memory are not atomic, and thus do not guarantee synchronization. However, there are alternatives to semaphores that are a bit simpler to use.

[@Neuroszima]

@@JacobSchrum well yeah, i was trying to understand the differences but i guess i have more reading to do. Thanks for pointing an unobvious thing to me

[@penguin1714]

I googled this video so I could know if I was saying it wrong.

[@grzechupo9199]

Thank you

[@sioudiheb5402]

Gold blessYou

[@Engineerblogbihar]

good

[@karinr7656]

thank you :)

[@ZiiiP2142]

Cheers.

[@mohamed_ibrahim07]

thanks!!!

[@Nwolf1840]

Thank god, I found an American.

[@sercantor1]

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