4 ms·

I still haven’t seen a good comparison between Futex and Benaphore. Benaphores I understand, it predates Futexes by almost a decade, but what do Futexes add to

I still haven’t seen a good comparison between Futex and Benaphore. Benaphores I understand, it predates Futexes by almost a decade, but what do Futexes add to the equation since hardly anyone talks about Benaphores (or is it a case of not invented here)?

the_mitsuhiko 1y ago
Other then avoiding a syscall on an uncontended path they are not really similar. A benaphore is just a semaphore with an extra atomic counter in userspace to count waiters. You can’t really use semaphores to implement things that can’t mutexes or semaphores so the overall utility is limited compare to futexes that you can use for condvars and other primitives too.
- krackers 1y ago
  i mean you can use semaphores to implement condition variables, but it's extremely tricky to the point where you've got to be Mike Burrows in order to do it right [1] (in fact, that's how it's done in the nsync library, as a fallback when every faster option is unavailable). [1] http://birrell.org/andrew/papers/ImplementingCVs.pdf http://birrell.org/andrew/papers/ImplementingCVs.pdf
  - the_mitsuhiko 1y ago
    > i mean you can use semaphores You absolutely can use semaphores for it, but the benefits of a benaphore would not work with when you build a condition variable with it. The wait on the CV would need to go through the kernel.
garaetjjte 1y ago
Benaphore is a kernel synchronization object behind atomic counter for uncontended path. But the kernel object needs to be always here, initialized and destroyed when appropriate. Futex doesn't need any kernel initialization, and from perspective of kernel it doesn't exist at all when there are no waiters. (see also CRITICAL_SECTION, which originally always had kernel object created with it, but it was later changed to create them on-demand falling back to keyed event on failure. SRWLock only uses keyed events. Keyed event only needs one object per process, and otherwise consumes kernel resources only if there are any waiters.)
tialaramex 1y ago
As others have explained the Benaphore is a speed-up for an existing OS primitive, but the futex is a new primitive that's often much better suited to the problem you have. Benoit Schillings uses this name "Benaphore" but never claims explicitly to have invented it in the article naming it, either way though Benoit worked for Be Inc. which were making an entire OS including its kernel, so they could have provided the better primitive. But they didn't, BeOS provided the limited semaphore primitive you'd have seen in a typical 1980s or 1990s Unix. Given that primitive, the Benaphore is a good way to use it, like if you've got a 1930s refrigerator and you've got a clever technique to reduce frost build-up - a modern fridge has a smarter controller and so it'll just defrost itself anyway automatically, no sweat. The Benaphore is thus redundant today - like that anti-frost technique for your 90 year old fridge.

the_mitsuhiko 1y ago

Other then avoiding a syscall on an uncontended path they are not really similar. A benaphore is just a semaphore with an extra atomic counter in userspace to count waiters. You can’t really use semaphores to implement things that can’t mutexes or semaphores so the overall utility is limited compare to futexes that you can use for condvars and other primitives too.

krackers 1y ago
i mean you can use semaphores to implement condition variables, but it's extremely tricky to the point where you've got to be Mike Burrows in order to do it right [1] (in fact, that's how it's done in the nsync library, as a fallback when every faster option is unavailable). [1] http://birrell.org/andrew/papers/ImplementingCVs.pdf http://birrell.org/andrew/papers/ImplementingCVs.pdf
- the_mitsuhiko 1y ago
  > i mean you can use semaphores You absolutely can use semaphores for it, but the benefits of a benaphore would not work with when you build a condition variable with it. The wait on the CV would need to go through the kernel.

garaetjjte 1y ago

Benaphore is a kernel synchronization object behind atomic counter for uncontended path. But the kernel object needs to be always here, initialized and destroyed when appropriate. Futex doesn't need any kernel initialization, and from perspective of kernel it doesn't exist at all when there are no waiters. (see also CRITICAL_SECTION, which originally always had kernel object created with it, but it was later changed to create them on-demand falling back to keyed event on failure. SRWLock only uses keyed events. Keyed event only needs one object per process, and otherwise consumes kernel resources only if there are any waiters.)

tialaramex 1y ago

As others have explained the Benaphore is a speed-up for an existing OS primitive, but the futex is a new primitive that's often much better suited to the problem you have. Benoit Schillings uses this name "Benaphore" but never claims explicitly to have invented it in the article naming it, either way though Benoit worked for Be Inc. which were making an entire OS including its kernel, so they could have provided the better primitive. But they didn't, BeOS provided the limited semaphore primitive you'd have seen in a typical 1980s or 1990s Unix. Given that primitive, the Benaphore is a good way to use it, like if you've got a 1930s refrigerator and you've got a clever technique to reduce frost build-up - a modern fridge has a smarter controller and so it'll just defrost itself anyway automatically, no sweat. The Benaphore is thus redundant today - like that anti-frost technique for your 90 year old fridge.