What would be a realworld example of use of a barrier in a multithreaded application?

Question

JDK's concurrency package, Boost's thread library, Perl's Thread library (not in Python though) all implement barrier, I haven't come across a need for using a barrier, so wondering what would a typical use case be in multi-threaded applications.

Answer 1

Barriers can be used all over the place through contrived examples but you'll often seem them in a scatter/reduce method where the results of the different threads are all required before proceeding.

If you wanted to parallelize a sort for instance you could split the list n times and start n threads to sort their section and pause, when they're all finished they would die letting the parent know that it's finally okay to combine the sorted chunks. (I know there are better ways but it's one implementation).

The other place I've seen it is in parallelized networking where you have to send a certain amount of data per payload. So the interface will start up n buckets and wait for them all to fill before sending out the transmission. When you think about a partitioned T1 line it sorta makes sense, sending one burst of data over the 64 multiplexed partitions would be better than sending 1 partition's data (which essentially costs the same since the packet has to be padded with 0's.)

Hope those are some things to get you thinking about it the problem!

Answer 2

Example: a set of threads work concurrently to compute a result-set and the said result-set (in part/total) is required as input for the next stage of processing to some/all threads at the "barrier".

A barrier makes it easier to synchronize multiple threads without having to craft a solution around multiple conditions & mutexes.

I can't say I have seen barriers often though. At some point, as the number of threads grows, it might be worthwhile considering a more "decoupled" system as to manage possible dead-locks.

Answer 3

MSDN: A Barrieris an object that prevents individual tasks in a parallel operation from continuing until all tasks reach the barrier. It is useful when a parallel operation occurs in phases, and each phase requires synchronization between tasks.

Found here