* block-unrolled scan implementation, via optional `_unroll` scan parameter
* index statically in the inlined path of lax.scan
* make `unroll` a required scan parameter, and test that it unrolls
For a computation of the form:
>>> f = lambda x: x ** 2
>>> f = jax.jit(f)
>>> while run:
... x = f(x)
JAX must currently always have two copies of `x` in device memory since there
is no reliable way in Python to determine whether there will be future uses of
`x`. This causes two classes of problem:
1. Users at the limit of available device are constrained by the additional
copy of their parameters and other state while they typically only require
one copy. This typically frees 100M+ of device memory and is a critical
optimization for larger models to match state of the art performance in
other frameworks.
2. This constant alloc/free of the input/output buffers can cause memory
fragmentation on some platforms (although having a reusing allocator and
limiting run-ahead may be a better solution for this problem).
We propose fixing this by using input/output aliasing as supported by XLA. We
will support this in JAX by allowing certain arguments of jit/pmap decorated
functions to be donated and reused as outputs:
>>> f = lambda x: x ** 2
>>> f = jit(f, donate_argnums=0)
>>> while run:
... x = f(x)
JAX will determine that the donated input `x` can alias with the output of the
function and it will instruct XLA it _must_ write the result to this buffer.
If a user tries to reuse a buffer after it has been donated they get an error
that the buffer is invalid:
>>> y = f(x)
>>> jax.device_get(x)
...
RuntimeError: Invalid argument: CopyToHostAsync() called on invalid buffer.
The semantics of `donate_argnums` follows that of `static_argnums`, namely that
it identifies positional arguments to the computation that are to be donated
to the computation and used as part of the output.
One feature that is also enabled by this is invalidating buffers that should
only be used once, for example PRNGKeys:
>>> @partial(jit, donate_argnums=0)
... def move(x):
... # Do something complex enough for JAX to just optimize it away.
... return tree_map(lambda x: x + x - x, x)
>>> def safe_eager_uniform(key, *a, **k):
... assert hasattr(key, 'device_buffer'), "random must run eagerly"
... key = move(key)
... return jax.random.uniform(key, *a, **k)
This is not a complete answer to random safety since it is still possible to
reuse a key as part of a traced computation, however it can be used to support
this feature (somewhat inefficiently) in eager mode.
* support replica groups in allreduce collectives
* add test and fix jaxpr in docs
* switch from XLA replica IDs to JAX axis indices
* fix psum transpose rule
* test other nesting order + imperfect nesting
* update jaxpr.rst
* handle None case
* add note+check that groups cover the index space
* switch split_axis assert to NotImplementedError
* update CHANGELOG
