Question for the coroutine experts here If I have a program

Question

Question for the coroutine experts here If I have a program that doesn t really do much blocking like let s say a single run of the program just does a bunch of math and local data collection transformations and I run that program again and again as fast as possible would there be much call for coroutines I am thinking not rather that just having a fixed set of threads 1 per core should be as effective I m thinking that coroutines really shine when there s significant blocking is that correct

TwoClocks · Answer

For most things ideally you want to run as many threads as you have CPUs maybe 1 Maybe more true for more CPU intensive things and maybe less true for other things but a general rule of thumb

TwoClocks · Answer

coroutines can be helpful in dispatching work to threads CPUs but if you already have a simple means to do that then yeah not sure it s buying you much

Byron Katz · Answer

helpful because lots of blocking actions give us an opportunity to be doing something rather than waiting And without which coroutines don t buy much

TwoClocks · Answer

For me the major bang for the buck in coroutines is the ability to write asynchronous code that looks synchronous which has nothing to do with threads really everything else launchers dispatchers channels etc is a bunch of syntactic sugar on top of threads Maybe kinda useful I suppose but not to me wink

Byron Katz · Answer

But it all gets back to blocking code doesn t it You write async because of blocking code don t you

Byron Katz · Answer

err I mean code that blocks and incurs delays

TwoClocks · Answer

your choices for blocking core are 1 block the thread 2 have a callback

TwoClocks · Answer

coroutines are a compiler trick to make 2 look like 1

TwoClocks · Answer

coroutines = threads It s just a way to hide callbacks

Byron Katz · Answer

That helps thank < TwoClocks>

Byron Katz · Answer

By the way why do you say to run number of processors minus one

TwoClocks · Answer

you can add threading on top of that but you can add threading on top of any fucntion call

TwoClocks · Answer

it really depends on what your doing exactly but sometimes you leave a CPU for the OS JVM to do to work

TwoClocks · Answer

on modern CPUs context switches are increadbly slow like really rreally rreally slow It takes less time to send a packet to another machine than for a CPU to context switch

TwoClocks · Answer

So if the JVM OS decide it s time to do some work and context switch your code out the results are bad

Byron Katz · Answer

You ve seen empirical evidence of that

TwoClocks · Answer

so you leave them a free CPU so they don t destory the nice warm cache you have going in your CPU

TwoClocks · Answer

what The context switch oh god yea it s a really easy test Just get two machine and do a ping pong test vs a thread switching test I do it on every new rev of CPU and it s just gotten slower over the past 10 years

Byron Katz · Answer

I ask just because when I run with cores == threads my performance graph looks very even for a long while If what you re saying is true wouldn t I see spikes in the graph

TwoClocks · Answer

it would depend on how cache effective your code is e g what is the damage when the cache is blown out and how long running your code is etc that kinda stuff is very code dependent

Byron Katz · Answer

I see Cool good insight

TwoClocks · Answer

it also depends on how you measure it are you measuing runtime or some inner loop cycle time