3 ms·

Had a mentor who said one of the biggest problems in computer science is that all the good metaphors are already taken.

dataflow 6y ago
But in this case the concept already had a name in CS and it was called a barrier (which is a pretty apt name too). For some rename they called it a latch and then they called a cyclic barrier a barrier...
- gpderetta 6y ago
  As far as I can tell, a (countdown) latch is different from a barrier. In a barrier signaling and waiting is a single atomic operation. Each thread (of a group of N) reaching a barrier will wait until all N threads have reached an waited (and implicitly signaled) it. A countdown latch is an event that will release one (or more) waiter only after has been signaled N times. Signaling and waiting threads are not necessarily the same and often are distinct sets. I guess you could build a barrier from a countdown latch, but I suspect that a trivial mapping of barrier::wait to latch::signal+wait is going to racey and you need an additional sinchronization primitive. For example pthread_barrier_wait requires an additional mutex (similar to condition variables). edit: std::barrier has (optionally) separate singal+wait and uses an explicit arrival token instead of a mutex to tie the signaling with waiting. In practice a barrier and a countdown latch are used for different purposes (the former to coordinate multiple symmetric threads across phases of a distributed computation, the latter to wait for completion of N events).
  - dataflow 6y ago
    > In a barrier signaling and waiting is a single atomic operation. Each thread (of a group of N) reaching a barrier will wait until all N threads have reached an waited (and implicitly signaled) it. Yeah that's exactly what std::latch::arrive_and_wait does. I don't think atomicity is a thing here though... arrive_and_wait() is just count_down() followed by wait(). > A countdown latch is an event that will release one (or more) waiter only after has been signaled N times. Signaling and waiting threads are not necessarily the same and often are distinct sets. C# still calls this a Barrier, except that it requires one of the waiters to be a participant (see AddParticipant() and SignalAndWait()): https://docs.microsoft.com/en-us/dotnet/api/system.threading.barrier?view=net-5.0 https://docs.microsoft.com/en-us/dotnet/api/system.threading... They even allow removing participants! This is a little bit like a mutex that has a try_lock. It's not strictly the vanilla Computer Science "mutex" with lock/unlock per se, but it's not a fundamentally different concept; it's just a handy yet pretty close generalization. I guess if you really want to give this a new name then maybe it's not a terrible idea given it's a slight generalization of a barrier, but latch is certainly not going to be any more accurate (or less confusing) than barrier.
    - gpderetta 6y ago
      Yes, if you can't reset the latch, I don't think there is actually any race. I didn't realize that latch had an arrive and wait in fact.
bierjunge 6y ago
I don't know who said the following, but it sums up CS pretty good: "There are two hard things in computer science: cache invalidation, naming things, and off-by-one errors."
MaxBarraclough 6y ago
It's interesting how we've used all the various 'doing' words in English for different technical concepts, although with some overlap. Function, method, procedure, action, operation, routine, task, process.

But in this case the concept already had a name in CS and it was called a barrier (which is a pretty apt name too). For some rename they called it a latch and then they called a cyclic barrier a barrier...

gpderetta 6y ago
As far as I can tell, a (countdown) latch is different from a barrier. In a barrier signaling and waiting is a single atomic operation. Each thread (of a group of N) reaching a barrier will wait until all N threads have reached an waited (and implicitly signaled) it. A countdown latch is an event that will release one (or more) waiter only after has been signaled N times. Signaling and waiting threads are not necessarily the same and often are distinct sets. I guess you could build a barrier from a countdown latch, but I suspect that a trivial mapping of barrier::wait to latch::signal+wait is going to racey and you need an additional sinchronization primitive. For example pthread_barrier_wait requires an additional mutex (similar to condition variables). edit: std::barrier has (optionally) separate singal+wait and uses an explicit arrival token instead of a mutex to tie the signaling with waiting. In practice a barrier and a countdown latch are used for different purposes (the former to coordinate multiple symmetric threads across phases of a distributed computation, the latter to wait for completion of N events).
- dataflow 6y ago
  > In a barrier signaling and waiting is a single atomic operation. Each thread (of a group of N) reaching a barrier will wait until all N threads have reached an waited (and implicitly signaled) it. Yeah that's exactly what std::latch::arrive_and_wait does. I don't think atomicity is a thing here though... arrive_and_wait() is just count_down() followed by wait(). > A countdown latch is an event that will release one (or more) waiter only after has been signaled N times. Signaling and waiting threads are not necessarily the same and often are distinct sets. C# still calls this a Barrier, except that it requires one of the waiters to be a participant (see AddParticipant() and SignalAndWait()): https://docs.microsoft.com/en-us/dotnet/api/system.threading.barrier?view=net-5.0 https://docs.microsoft.com/en-us/dotnet/api/system.threading... They even allow removing participants! This is a little bit like a mutex that has a try_lock. It's not strictly the vanilla Computer Science "mutex" with lock/unlock per se, but it's not a fundamentally different concept; it's just a handy yet pretty close generalization. I guess if you really want to give this a new name then maybe it's not a terrible idea given it's a slight generalization of a barrier, but latch is certainly not going to be any more accurate (or less confusing) than barrier.
  - gpderetta 6y ago
    Yes, if you can't reset the latch, I don't think there is actually any race. I didn't realize that latch had an arrive and wait in fact.

bierjunge 6y ago

I don't know who said the following, but it sums up CS pretty good: "There are two hard things in computer science: cache invalidation, naming things, and off-by-one errors."

MaxBarraclough 6y ago

It's interesting how we've used all the various 'doing' words in English for different technical concepts, although with some overlap. Function, method, procedure, action, operation, routine, task, process.