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It was a deliberate choice, but it was a poor one. It made implementing a fast cpu easier, but it also made the consistency model very hard for programmers to r

It was a deliberate choice, but it was a poor one. It made implementing a fast cpu easier, but it also made the consistency model very hard for programmers to reason about, and required much more explicit barriers than any other consistency model. 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. Had Alpha not died early, they would absolutely have eventually moved towards a more strict memory model. As it was, it was essentially an irrelevant architecture by the time people started really hitting all the pitfalls.

_a_a_a_ 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. > 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.

_a_a_a_ 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. > 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.