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I was under the impression that Ocaml was single threaded? How is it that they have type checker that runs in parallel?

by leshow 8y ago

verlaguet 8y ago
It’s because it is built on top of the infrastructure that was introduce for Hack. The system first starts by allocating a large area in shared memory (through a call to mmap). That area is very large but because most of it wont be written to it’s ok in practice. After that, the program forks as many times as there are cores. Each of those cores are called “workers”. The first program is now the master. The master and the workers communicate through pipes, but that is only used for synchronization. The lion share of the data goes through that shared memory that was mmaped at the beginning. There are 2 main things shared in that area. 1- atomic hashtables of serialized ocaml objects (you can define as many as you like with a functor) 2- an atomic table of dependencies. Each workers can read and write to those tables without locks (but only the master can remove from them). It turns out that that setup works well in practice. Serializing/deserializing costs are mitigated by a cache of deserialized values for each core. And this way each core can manage there memory (the gc does not need to scan the shared memory). Because that setup was working well in practice, it was reused by Flow and now Pyre. We refer to that setup internally as the “Hack infrastructure”.
sinancepel 8y ago
You can have multi-threaded OCaml (our server actually has two threads), but only one can run at a time currently. We solve this problem by having a multi-process architecture where workers only communicate through a lock-free hash table in shared memory.
- leshow 8y ago
  Is it fair to say that it runs in parallel if only one thread may run at a time? Isn't that concurrent, but not parallel?
  - platinumrad 8y ago
    Both responses to your original question mention that multiple processes are run.
    - deleted 8y ago
      [deleted]

I was under the impression that Ocaml was single threaded? How is it that they have type checker that runs in parallel?

by leshow 8y ago

I was under the impression that Ocaml was single threaded? How is it that they have type checker that runs in parallel?

It’s because it is built on top of the infrastructure that was introduce for Hack. The system first starts by allocating a large area in shared memory (through a call to mmap). That area is very large but because most of it wont be written to it’s ok in practice. After that, the program forks as many times as there are cores. Each of those cores are called “workers”. The first program is now the master. The master and the workers communicate through pipes, but that is only used for synchronization. The lion share of the data goes through that shared memory that was mmaped at the beginning. There are 2 main things shared in that area. 1- atomic hashtables of serialized ocaml objects (you can define as many as you like with a functor) 2- an atomic table of dependencies. Each workers can read and write to those tables without locks (but only the master can remove from them). It turns out that that setup works well in practice. Serializing/deserializing costs are mitigated by a cache of deserialized values for each core. And this way each core can manage there memory (the gc does not need to scan the shared memory). Because that setup was working well in practice, it was reused by Flow and now Pyre. We refer to that setup internally as the “Hack infrastructure”.

sinancepel 8y ago

You can have multi-threaded OCaml (our server actually has two threads), but only one can run at a time currently. We solve this problem by having a multi-process architecture where workers only communicate through a lock-free hash table in shared memory.

leshow 8y ago
Is it fair to say that it runs in parallel if only one thread may run at a time? Isn't that concurrent, but not parallel?
- platinumrad 8y ago
  Both responses to your original question mention that multiple processes are run.
  - deleted 8y ago
    [deleted]