Rethinking the futex API
Rethinking the futex API
Posted Jun 20, 2020 3:58 UTC (Sat) by itsmycpu (guest, #139639)In reply to: Rethinking the futex API by krisman
Parent article: Rethinking the futex API
> in order to wake all of the entries in its waitqueue, it walks the list and does a FUTEX_WAKE syscall per entry. That's much worse than FWM.
Almost, but not with that disadvantage. As I indicated, "in my design" there is (optionally) a FUTEX_WAKE_MULTIPLE (not wait_multiple) kernel API which is much simpler to implement, also because it doesn't need to copy and maintain the list internally in the kernel. It does multiple FUTEX_WAKE operations in a single syscall. And this can be added later on, as an optimization, if and when such a call becomes available in the kernel, while the rest can already be implemented, tested and tuned with the kernel as is.
Since a FUTEX_WAKE is only needed for futexes that actually have thread(s) waiting, and the waiting itself is already a time consuming kernel call, one non-blocking WAKE call per waiting futex shouldn't be that bad, although it will be better to have a wake_multiple call, which solves that problem (if it is one) completely.