3 ms·

IIRC alpha-model memory barriers are still used in the linux kernel. That said, I can't find a clear statement of that so I don't know if it is true or was, or

IIRC alpha-model memory barriers are still used in the linux kernel. That said, I can't find a clear statement of that so I don't know if it is true or was, or just my own memory. > These barriers also meant that correct multi-threaded alpha code was no longer particularly fast, because you have to insert expensive memory barriers basically everywhere. I don't buy it. MBs are for multi-core code, and in such code you typically do much work on a single core then have a quick chat with another core. So the MBs are there for the inter-core chatter only. In that case having fast monocore code is a big win.

jabl 4y ago
> IIRC alpha-model memory barriers are still used in the linux kernel. That said, I can't find a clear statement of that so I don't know if it is true or was, or just my own memory. The various memory barriers and locking primitives are arch-specific code, and at least smp_read_barrier_depends() is a no-op on all architectures except Alpha. Apparently around the 4.15-4.16 kernels there was a bit of de-Alphafication going on which entailed removing much Alpha-specific code from core kernel code. Further in 5.9 {smp_,}read_barrier_depends() were removed from the core barriers, at the cost of making some of the remaining memory barriers on Alpha needlessly strong. For more info, search for Alpha e.g. in https://www.kernel.org/doc/Documentation/memory-barriers.txt https://www.kernel.org/doc/Documentation/memory-barriers.txt https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin... https://open-std.org/JTC1/SC22/WG21/docs/papers/2020/p0124r7.html https://open-std.org/JTC1/SC22/WG21/docs/papers/2020/p0124r7... https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook.2022.09.25a.pdf https://mirrors.edge.kernel.org/pub/linux/kernel/people/paul...
- _a_a_a_ 4y ago
  Another cracking answer, I'll have a dive in this evening, thanks.

jabl 4y ago

> IIRC alpha-model memory barriers are still used in the linux kernel. That said, I can't find a clear statement of that so I don't know if it is true or was, or just my own memory. The various memory barriers and locking primitives are arch-specific code, and at least smp_read_barrier_depends() is a no-op on all architectures except Alpha. Apparently around the 4.15-4.16 kernels there was a bit of de-Alphafication going on which entailed removing much Alpha-specific code from core kernel code. Further in 5.9 {smp_,}read_barrier_depends() were removed from the core barriers, at the cost of making some of the remaining memory barriers on Alpha needlessly strong. For more info, search for Alpha e.g. in https://www.kernel.org/doc/Documentation/memory-barriers.txt https://www.kernel.org/doc/Documentation/memory-barriers.txt https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin... https://open-std.org/JTC1/SC22/WG21/docs/papers/2020/p0124r7.html https://open-std.org/JTC1/SC22/WG21/docs/papers/2020/p0124r7... https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook.2022.09.25a.pdf https://mirrors.edge.kernel.org/pub/linux/kernel/people/paul...

_a_a_a_ 4y ago
Another cracking answer, I'll have a dive in this evening, thanks.