> I think you might be talking about "false sharing." This is real contention on the cache line due to co-location of apparently unrelated variables.
You’re right. And cache lines are quite small, so this is probably less common. Yet, it’s another potential source of perf regressions in concurrent code, as if it wasn’t incredibly complex already.
> Ehh.. only the hot ones matter.
Well.. first atomics have even more non-local effects, such as barriers on instruction reordering. So Arcs that are cloned willy nilly can still be significant, with no contention.
But let’s ignore that and focus on the contended case: when you hear “uncontended X are basically free” it (subjectively, imo) downplays the issue, like contention is some special case that you can compartmentalize and only worry about when you consciously decide to write contended code. The blog post demonstrates exactly how this is so easy for contention to creep in, that you have to be superhuman levels of vigilant and paranoid to spot these issues upfront. Extremely easy to miss in eg code review.
I think both compile- and runtime tooling could help at least partly here. I’d also give rust some credit for having explicit clone instead of hiding it.
You’re right. And cache lines are quite small, so this is probably less common. Yet, it’s another potential source of perf regressions in concurrent code, as if it wasn’t incredibly complex already.
> Ehh.. only the hot ones matter.
Well.. first atomics have even more non-local effects, such as barriers on instruction reordering. So Arcs that are cloned willy nilly can still be significant, with no contention.
But let’s ignore that and focus on the contended case: when you hear “uncontended X are basically free” it (subjectively, imo) downplays the issue, like contention is some special case that you can compartmentalize and only worry about when you consciously decide to write contended code. The blog post demonstrates exactly how this is so easy for contention to creep in, that you have to be superhuman levels of vigilant and paranoid to spot these issues upfront. Extremely easy to miss in eg code review.
I think both compile- and runtime tooling could help at least partly here. I’d also give rust some credit for having explicit clone instead of hiding it.