40K views · Oct 8, 2018 · 1:04:26 · 0 HN recommenders · 3 mentions · discussed 2018–2021
Gor Nishanov's CppCon 2018 talk on using tiny C++ coroutines to hide memory latency, interleaving useful work while prefetches bring cache lines in from main memory.
Why Hacker News points people here: Cited as a deep dive on coroutines for cache misses and on running huge numbers of coroutines without heap allocation.
“also see this video that goes in depth how to have billions of coroutines with C++”HN comment, 2019
“jump to ~12:33 to skip background”
Coroutines with prefetch to hide cache misses.
Gor Nishanov, one of the authors, talks about this in his CppCon2018 talk this video (jump to ~12:33 to skip background). It amounts to using CPU prefetch instructions with C++ coroutines to simulate hyperthreading in software by scheduling instructions around cache misses (but is potentially better than hyperthreads because it's not limited to 2/core)
“also see this video that goes in depth how to have billions of coroutines with C++”
Deep dive on running billions of C++ coroutines without heap allocation.
> In C++, technically every initialization of a sub-coroutine starts defaults to being a new heap allocation. I don't want to spread FUD: my understanding is that the vast majority of these are optimized out by the compiler. But one downside of this approach is that you could change your code and accidentally disable one of these optimizations. I don't think this is correct. C++ 20 allows a lot of choices to implement it without forcing a heap allocation. see link also see this video that goes in depth how to have billions of coroutines with C++: this video
“The example given”
Example of coroutines issuing prefetches around memory latency.
It's a big deal because, while it has some downsides, being stalkless means they can have next to no overhead, meaning it can be performant to use coroutines to write asynchronous code for even very fast operations. The example given this video is that you can launch multiple coroutines to issue prefetch instructions and process the fetched data, so you can have clean code that issues multiple prefetches and process the results. Whereas in Python (don't get me wrong, I love Python) you might use a generator to "asynchronize" slow operations like requesting and processing data from remote servers, C++ coroutines can be fast enough to asynchronously process "slow" operations like requesting data from main memory.
