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[GitHub] [tvm-rfcs] JosephTheOctonaut commented on a diff in pull request #80: Asynchronous stage in software pipeline
JosephTheOctonaut commented on code in PR #80:
URL: https://github.com/apache/tvm-rfcs/pull/80#discussion_r904223269
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rfcs/0077-async-pipeline.md:
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+- Feature Name: Asynchronous stage in software pipeline
+- Authors: [Masahiro Masuda](https://github.com/masahi), [Wuwei Lin](https://github.com/vinx13/)
+- Start Date: (2022-06-17)
+
+# Summary
+This RFC proposes two TIR intrinsics and an additional annotation to the TIR software pipeline transform, to express asynchrony **within the device code**.
+Asynchrony is prevalent on the host (runtime) side, and this proposal is the first step toward bringing the notion of an asynchronous operation in the
+generated code.
+
+The most important component we should agree on is the model of synchronization: Coming up with a design that is general enough to be useful for diverse backends, while making sure that the chosen design can be translated to a low-level synchronization model of a particular backend, is highly non-trivial.
+The approach described in this document is motivated by a use case for NVIDIA GPUs, but we took some cares so that the design can be adopted by other backends. For example, if a backend has an asynchronous DMA engine, vector and tensor unit, we can specify that each of them runs asynchronously in different stages in a pipeline, with necessary synchronization between them.
+
+The proposed model may have diverged from conventional ones, but we believe that this is a good fit for the TIR software pipeline specifically.
+
+# Asynchronous stage in a software pipeline
+
+### Background: What is a software pipeline, and what does the TIR software pipeline transform do?
+
+Software pipeline is an optimization technique to improve instruction-level parallelism of a loop. For example, given this program:
+
+```python
+B = alloc([1])
+
+for i in range(16):
+ B[0] = A[i] + 1
+ C[i] = B[0] + 1
+```
+
+the goal is to overlap the execution of two statements in the loop body, by letting the two statements operate on different iterations of the loop. This way, the second statement would no longer depend on the completion of the first statement in the same iteration.
+
+The TIR software pipeline transform enables such transformation at the TIR level. We annotate the loop in the above program to specify, for each statement in the loop, the “stage” and the “order” in the pipeline:
+
+```python
+sch = ...
+sch.annotate(i, "software_pipeline_stage", [0, 1])
+sch.annotate(i, "software_pipeline_order", [0, 1])
+```
+
+Given the annotation above, the TIR software pipeline transform would break up the loop into three parts: prologue, pipeline body and epilogue. Different “stage” in the pipeline body become independent of each other, and the integer value of “stage” tells how many iterations each statement goes ahead of its consumer.
+
+```python
+B = alloc([2])
+
+# Prologue
+B[0] = A[0]
+
+# Body
+for i in range(15):
+ B[(i + 1) % 2] = A[i] + 1
+ C[i] = B[i % 2] + 1
+
+# Epilogue
+C[15] = B[1] + 1
+```
+
+The two statements in the body can potentially run in parallel, if the underlying HW supports out-of-order execution.
+
+### Making parallelism more explicit: Asynchronous pipeline
+
+What’s currently available today is, after all, a “software” pipeline: whether or not independent statements actually run in parallel is up to the underlying HW, and programmers have little control over it. Moreover, for in-order processors like Hexagon DSP, this transformation alone would not help.
+
+The goal of this work is to support HW-backed asynchrony in the pipeline. Asynchronous data movement is becoming increasingly important in GPU computing, and NPUs typically have multiple kinds of asynchronous units (DMA copy, vector & matrix compute etc). To exploit such hardware features, it’s essential that we express all kinds of available asynchronies in the IR.
+
+A user of the TIR software pipeline transform will be able to specify which data movement or compute block should become asynchronous by an additional annotation. For example, given the annotation specifying that the first block in the pipeline be made async,
+
+```python
+for i in range(16):
+ B[0] = A[i] + 1
+ C[i] = B[0] + 1
+
+...
+sch.annotate(i, "software_pipeline_stage", [0, 1])
+...
+
+# "0" refers to the first element in te list [0, 1] above, i.e. the first block
+sch.annotate(i, "software_pipeline_async_stages", [0])
+```
+
+we generate the following IR. An asynchronous block is decorated with the `async_scope` attribute, and two intrinsics are inserted to express synchronization.
+
+```python
+B = alloc([2])
+
+# Prologue
+async_scope:
+ B[0] = A[0]
+ async_commit_stage(0)
+
+# Body
+for i in range(15):
+ async_scope:
+ B[(i + 1) % 2] = A[i] + 1
+ async_commit_stage(0)
+
+ async_wait_stage(0, 1)
+ C[i] = B[i % 2] + 1
+
+# Epilogue
+async_wait_stage(0, 0)
+C[15] = B[1] + 1
+```
+
+**Semantics of the proposed intrinsics**. “stage” refers to the same notion in the TIR software pipeline.
+- `async_commit_stage(i)` : Group one or more invocation of async operations, and “commit” them to the `i`-th stage. The exact interpretation of “committing” can be up to each backend, but informally it signifies that a group of async operations are now in-flight. The group of operations committed together is awaited as one chunk, and thus they constitute the granularity at which the synchronization intrinsic discussed next operates on.
+- `async_wait_stage(i, N)` : Block until only `N` **most recent** committed groups are still in-flight at the stage `i` . In other words, if there are `M` committed groups in-flight at the stage `i`, at the invocation of `async_wait_stage(i, N)`, `M - N` oldest committed groups would be forced to complete. `N` doesn’t have to be a constant, but some backends may require a constant count (e.g. PTX)
+
+They directly correspond to the async data movement instructions in CUDA (PTX): [`cp.async.commit_group`](https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#data-movement-and-conversion-instructions-cp-async-commit-group) and [`cp.async.wait_group`](https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#data-movement-and-conversion-instructions-cp-async-wait-group).
+
+The CUDA counterparts do not have the notion of “stage”, since there is only one kind of async operation (copy from global to shared memory) supported by the current generation of NVIDIA GPU (Ampere, at the time of writing). To support more general cases where there could be multiple kinds of async “engine”, each of which corresponds to a different stage in an async pipeline, TIR `async_commit_stage` and `async_wait_stage` take a “stage” parameter.
+
+**The role of async_scope**. `async_scope` is represented by `AttrStmt` with key `tir::attr::async_scope`. It is inserted to let later transform passes know that the enclosed statement is intended to run asynchronously. This way, the actual lowering to target-dependent asynchronous instructions
+can happen much later in the compilation flow, rather than before the software pipeline transform using tensorization. For example, rewriting of global to shared memory copy by CUDA-specific `cp.async` can be made simpler if the rewrite happens after buffer flattening and loop vectorization passes.
+
+### `wait(in-flight-count)` vs `wait(finished-count)`
+
+ It would be more intuitive if the semantics of `wait(N)` was “Wait until the oldest N async operations have completed”. But that would make translation to the corresponding PTX instruction more complicated, since we additionally need to keep track of the “number of async operations in-flight” at each synchronization point, and make that an additional argument to `async_wait_stage` so that we can do subtraction during translation of `async_wait_stage` to `cp.async`.
+
+One of the pros of `wait(in-flight-count)` semantics is that, it is trivial to let all in-flight async operations to complete: `wait(0)`. The alternative semantics would require, again, precise tracking of the number of async operations in-flight at the desired sync point.
+
+
+### More examples
+
+**Three stages of compute, where the first two stages are async**. The second stage is both an async producer and consumer. This example demonstrates the use of the “stage” parameter. Note that there is no distinction of asynchronous copy or compute.
+
+```python
+B = alloc([1])
+C = alloc([1])
+
+for i in range(16):
+ B[0] = A[i] + 1
+ C[0] = B[0] + 1
+ D[i] = C[0] + 1
+```
+
+```python
+sch = ...
+sch.annotate(i, "software_pipeline_stage", [0, 1, 2])
+sch.annotate(i, "software_pipeline_order", [0, 1, 2])
+# The first and second statements are async, and they are in different stages
+sch.annotate(i, "software_pipeline_async_stages", [0, 1])
+```
+
+```python
+B = alloc([2])
+C = alloc([2])
+
+# Prologue
+for i in range(2):
+ async_scope:
+ B[i % 2] = A[i] + 1
+ async_commit_stage(0)
+
+ if 1 <= i:
+ async_wait_stage(0, 1)
+ async_scope:
+ C[(i - 1) % 2] = B[(i - 1) % 2] + 1
+ async_commit_stage(1)
+
+# Body
+for i in range(14):
+ # Stage 0
+ async_scope:
+ B[(i + 2) % 2] = A[i + 2] + 1
+ async_commit_stage(0)
+
+ # Stage 1
+ async_wait_stage(0, 1)
+ async_scope:
+ C[(i + 1) % 2] = B[(i + 1) % 2] + 1
+ async_commit_stage(1)
+
+ # Stage 2
+ async_wait_stage(1, 1)
+ D[i] = C[i % 2] + 1
+
+
+# Epilogue
+for i in range(2):
+ if i < 1:
+ async_wait_stage(0, 0)
+ async_scope:
+ C[(i + 15) % 2] = B[(i + 15) % 2] + 1
+ async_commit_stage(1)
+
+ if i < 1:
+ async_wait_group(1, 1)
+ else:
+ async_wait_group(1, 0)
+
+ D[(i + 14) % 2] = C[(i + 14) % 2] + 1
+
+```
+
+**Multi-stage pipelined GEMM where the shared memory copy is 4x multi-buffered + async, and shared to local copy is double-buffered**. This example uses a highly non-obvious annotation below and exercises the nested pipelining feature in the TIR software pipeline transform.
+
+```python
+sch.annotate(k0, ann_key="software_pipeline_stage", ann_val=[0, 0, 2, 3, 3])
+sch.annotate(k0, ann_key="software_pipeline_order", ann_val=[0, 1, 3, 2, 4])
+sch.annotate(k0, ann_key="software_pipeline_async_stages", ann_val=[0, 1])
+
+sch.annotate(k1, ann_key="software_pipeline_stage", ann_val=[0, 0, 1])
+sch.annotate(k1, ann_key="software_pipeline_order", ann_val=[0, 1, 2])
+```
+
+`async_commit_stage` is inserted after copies to `A_shared` and `B_shared` are issued, so that the two copies can be awaited as one chunk.
+
+```python
+
+# Prologue
+A_local = [2, ...]
+B_local = [2, ...]
+A_shared = [4, ...]
+B_shared = [4, ...]
+
+for i in range(3):
+ async_scope:
+ A_shared[i] <- global[...]
+
+ async_scope:
+ B_shared[i] <- global[...]
+
+ async_commit_stage(0)
+
+ if 2 <= i:
+ async_wait_stage(0, 2)
+ A_local[0] <- A_shared[0, ...]
+ B_local[0] <- B_shared[0, ...]
+
+# Body
+for i in range(125):
+ async_scope:
+ A_shared[(i + 3) % 4] <- global[...]
+
+ async_scope:
+ B_shared[(i + 3) % 4] <- global[...]
+
+ async_commit_stage(0)
+
+ async_wait_stage(0, 2)
+
+ A_local[1] <- A_shared[i % 4, ...]
+ B_local[1] <- B_shared[i % 4, ...]
+
+ compute(A_local[0], B_local[0])
+
+ A_local[0] <- A_shared[(i + 1) % 4, ...]
+ B_local[0] <- B_shared[(i + 1) % 4, ...]
+
+ compute(A_local[1], B_local[1])
+
+# Epilogue
+for i in range(3):
+ async_wait_stage(0, 1 - i)
+
+ A_local[1] <- A_shared[0, ...]
+ B_local[1] <- B_shared[0, ...]
+
+ compute(A_local[0], B_local[0])
+
+ if i < 2:
+ A_local[0] <- A_shared[0, ...]
+ B_local[0] <- B_shared[0, ...]
+
+ compute(A_local[1], B_local[1])
+
+```
+
+### Implicit vs explicit approach to synchronization
+
+The model of async synchronization adopted by CUDA can be categorized as an “implicit” one: Instead of saying “Wait for this operation to complete”, it says “Wait until only N most recent async operations are in flight”, or equivalently, “Wait until M oldest async operates have completed”, where M = “number of async operations in flight” - N.
+
+In contrast, a standard and intuitive approach is more explicit, e.g. wait for the operation associated with “this” token / future to complete etc. This is true for “async-await” in general-purpose languages, [Async](https://mlir.llvm.org/docs/Dialects/AsyncDialect/) and [NVGPU](https://mlir.llvm.org/docs/Dialects/NVGPU/) dialects in MLIR, for example.
+
+In general, the explicit approach is probably more preferable, since
+
+- It makes it obvious which operation is waiting on which
+- It is less stateful (less assumption on how the underlying HW should work)
+- It naturally handles synchronization in the presence of control flow (since we can only wait on an operation that has actually happened).
+
+These properties may help if we want do some analysis of async programs.
+
+The current design started from and has stayed with CUDA’s implicit synchronization model based on counting, primarily because it makes mapping to the corresponding PTX instructions trivial. We can adopt the explicit model instead, if we have a good way to translate token-based synchronization to the counting one for PTX. So far, we do not have a good solution for this. MLIR has adopted the token abstraction, but they have not solve this problem either: Their `DeviceAsyncWaitOp` has [an optional attribute `numGroups`](https://mlir.llvm.org/docs/Dialects/NVGPU/#nvgpudevice_async_wait-mlirnvgpudeviceasyncwaitop) that directly corresponds to "in-flight count", and they basically generate either `wait(numGroups)` or `wait(0)`, [in their translation](https://github.com/llvm/llvm-project/blob/main/mlir/lib/Conversion/NVGPUToNVVM/NVGPUToNVVM.cpp#L426-L427) of `DeviceAsyncWaitOp` (token based) to PTX `cp.async` (counting based). `wait(0)` is always correct but least precise / efficient.
+
+
+
+(The following is highly speculative) On the other hand, translation from “count” to “token” seems more feasible: At each synchronization point, a backend presumably maintains the number and the order of pending async operations. Given the count, it should be possible to derive the correct token from the corresponding ordered list of tokens.
Review Comment:
@masahi I believe I understand the intended usage of "stage" here, but _could_ we use it like Eric is suggesting? Is there a limitation to having an arbitrary number of async stages?
@Lunderberg Unless I've misunderstood, this translation would require fully unrolling all loops, right? Because each time you call `commit` you need to pass in a new static integer?
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