Reminds me of the era when dual-core processors started becoming generally available. Suddenly the bugs in multi-threaded software were much more apparent.
Vendors replied to complaints with: “We don’t support those processors”.
No buddy, you don’t support stable software. It’s buggy even on a single core, it’s just less obvious.
It's a very interesting phenomenon. There are lots of claims along the lines of "a single CPU is so fast it will exercise all kinds of multi-threading patterns very quickly" but in my experience it indeed takes multiple processors to reliably show up various races and faulty implicit orderings.
Single processors interleave threads using very coarse "quantums" of execution, where each thread runs for many milliseconds at a time. Dual processors will run the instruction streams truly in parallel.
From a correctness perspective, they're "identical", because even on a single core a context switch can occur anywhere. In theory, even one core can interleave individual instructions, it's just not done for performance reasons.
So if multiple cores causes a crash, then that crash bug was always there, and could have occurred even on a single core. It's just much less likely to be triggered.
Thankfully now every mainstream computer and practically all phones are multi-core, so software has to be written properly. The early 2000s were a mess though, with a lot of heisenbugs that were maddening to track down.
Comments
Reminds me of the era when dual-core processors started becoming generally available. Suddenly the bugs in multi-threaded software were much more apparent.
Vendors replied to complaints with: “We don’t support those processors”.
No buddy, you don’t support stable software. It’s buggy even on a single core, it’s just less obvious.
It's a very interesting phenomenon. There are lots of claims along the lines of "a single CPU is so fast it will exercise all kinds of multi-threading patterns very quickly" but in my experience it indeed takes multiple processors to reliably show up various races and faulty implicit orderings.
Single processors interleave threads using very coarse "quantums" of execution, where each thread runs for many milliseconds at a time. Dual processors will run the instruction streams truly in parallel.
From a correctness perspective, they're "identical", because even on a single core a context switch can occur anywhere. In theory, even one core can interleave individual instructions, it's just not done for performance reasons.
So if multiple cores causes a crash, then that crash bug was always there, and could have occurred even on a single core. It's just much less likely to be triggered.
Thankfully now every mainstream computer and practically all phones are multi-core, so software has to be written properly. The early 2000s were a mess though, with a lot of heisenbugs that were maddening to track down.