My understanding is that you cannot talk about warp specialization without talking about the alternative: multi-stage pipelining. And the final example code given is multi-stage pipeline with double buffers.
And here is my understanding where it differs:
1. multi-stage pipeline requires careful hand-tuning, even at PTX level to make sure your async wait is weaved properly to maximize overlap.
2. since these register files now is huge, multi-stage pipeline is difficult to write at intrinsics level to make efficient use of these huge register files.
3. Warp specialization delegated most of these scheduling dynamically, hence it is better adapted to hardware (and have more information to make scheduling decisions at runtime). Although this is a bit moot because we write different code for different hardware anyway.
Anything more I am missing?
rohany•4mo ago
Author here! I think that warp specialization is inherently related to multi-stage pipelining, they aren't really alternatives of each other. Warp specialization is a way to realize a multi-stage pipeline in the face of hazards that may cause the pipeline to spill out of the register file or not let parts of the pipeline run concurrently as desired.
The fact that we tend to need different warp specialization strategies for different hardware is a consequence of the capabilities of that hardware (i.e. different asynchronous instruction types), and contributes to the complexity of targeting that new hardware.
majke•4mo ago
I always assumed that when one warp waits for results from a long latency instruction, another warp, potentially from another block can be scheduled in.
I guess this post assumes the need to use all the gpu resources from within a single block.
rohany•4mo ago
> I always assumed that when one warp waits for results from a long latency instruction, another warp, potentially from another block can be scheduled in.
Yes, that is correct. However, most MMA-style kernels that utilize the Tensor Core usually need enough resources per block that only 1 block fits on each SM.
liuliu•4mo ago
And here is my understanding where it differs:
1. multi-stage pipeline requires careful hand-tuning, even at PTX level to make sure your async wait is weaved properly to maximize overlap.
2. since these register files now is huge, multi-stage pipeline is difficult to write at intrinsics level to make efficient use of these huge register files.
3. Warp specialization delegated most of these scheduling dynamically, hence it is better adapted to hardware (and have more information to make scheduling decisions at runtime). Although this is a bit moot because we write different code for different hardware anyway.
Anything more I am missing?
rohany•4mo ago
The fact that we tend to need different warp specialization strategies for different hardware is a consequence of the capabilities of that hardware (i.e. different asynchronous instruction types), and contributes to the complexity of targeting that new hardware.