I have seen that once in supposedly thread-safe C++ software in an industrial automation application. What happened was that it was somehow relying on the Windows UI messaging system (the message pump) which is single-threaded, and the fact that there were no multi-core CPUs when that stuff was written a long time ago. The latter has the effect that the CPU always sees a consistent view on the memory, no locks, mutexes and barriers needed.
Porting the thing to a multi-core system revealed that there were a lot of nasty concurrency bugs, like dead-locks or crashes which happened after a day of operation. And this wasn't a toy system - it was in use for a long time in an industrial application, and the customer was not too happy about the intermittent dead-locks. I commiserated with the poor engineer who had the quite stressful task to debug this, equipped with a lot of dedication but an insufficient background.
Frankly, while it would be nice to be able to write parallel code in pure Python, I think that Clojure with its purely-functional approach has the better concepts for this. And moreover, actually improving performance by parallel computation (using several CPUs to work in parallel on the same thing) is damn hard and unsolved in many cases (just come up with an efficient parallel Fast Fourier Transform and you might get a Turing award). What is mostly needed (outside of massive data processing pipelines) is concurrency for event-driven systems. Python can handle that, Clojure does handle it in a much more elegant way.
Porting the thing to a multi-core system revealed that there were a lot of nasty concurrency bugs, like dead-locks or crashes which happened after a day of operation. And this wasn't a toy system - it was in use for a long time in an industrial application, and the customer was not too happy about the intermittent dead-locks. I commiserated with the poor engineer who had the quite stressful task to debug this, equipped with a lot of dedication but an insufficient background.
Frankly, while it would be nice to be able to write parallel code in pure Python, I think that Clojure with its purely-functional approach has the better concepts for this. And moreover, actually improving performance by parallel computation (using several CPUs to work in parallel on the same thing) is damn hard and unsolved in many cases (just come up with an efficient parallel Fast Fourier Transform and you might get a Turing award). What is mostly needed (outside of massive data processing pipelines) is concurrency for event-driven systems. Python can handle that, Clojure does handle it in a much more elegant way.