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by tialaramex
1158 days ago
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BeOS, which is the foundation for most of this work, is from the 1990s. But in the 1990s C++ didn't yet have a clearly defined Memory Model for the purposes of concurrent algorithms and BeOS doesn't try to come up with one. In the subsequent 25+ years, C++ developed a Memory Model which distinguishes ordering for these atomic operations. Because BeOS pre-dates that, it doesn't document an ordering, let alone provide a mechanism to pick one. Unfortunately different concurrent algorithms have different ordering requirements, if what you're given is looser the algorithms may malfunction, if it's stricter the performance may be hampered. I reckon that before trying to claim you've innovated here it might be a good sense check to compare baseline. What exactly are the Haiku atomic operations, in terms of the C++ 11 Memory Model ? If you were Linux (or to some extent Windows) you could trail blaze here, because you're innovating before 2011, you're inventing the model - but this is 2023, so Haiku is off in the jungle on its own and everybody else has a map now, figure out where you are on that map first. |
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> I reckon that before trying to claim you've innovated here it might be a good sense check to compare baseline.
The baseline is "what are other operating systems' kernel- and userland-level condition variables APIs?" And none of the ones I looked at had anything like what Haiku has here, they all have something which is the more classical "lock-switched condvars" just like POSIX has.
The API itself does not depend on what memory ordering semantics are any more than a "mutex_lock()" API does. The implementation will be somewhat contingent on it, of course, but those are two separate matters.
> What exactly are the Haiku atomic operations, in terms of the C++ 11 Memory Model?
The atomic_() functions are (on most architectures, x86 included) implemented using GCC/Clang's __atomic_* functions, with various __ATOMIC_* orderings chosen as appropriate. You can see them defined in the system header here: https://github.com/haiku/haiku/blob/master/headers/os/suppor...
> because you're innovating before 2011, you're inventing the model
No, not really? GCC has had atomic builtins since at least 4.1.0 in 2006. The documentation (https://gcc.gnu.org/onlinedocs/gcc-4.1.0/gcc/Atomic-Builtins...) says: "In most cases, these builtins are considered a full barrier. That is, no memory operand will be moved across the operation, either forward or backward." -- which is basically equivalent to today's __ATOMIC_SEQ_CST.
> so Haiku is off in the jungle on its own and everybody else has a map now, figure out where you are on that map first.
We already did that years ago. The atomic_*() functions linked above in SupportDefs.h have been implemented using the C++11-standard GCC builtins since 2014, and the older __sync_* builtins for years before that.
Anyway, the algorithm described in this article, even if Haiku's atomic functions were not 1:1 with C++11-standard definitions (which they are, as noted above), is clearly portable to other OS kernels. So I am not sure what basis your comment has, regardless.