**Problem:**
Fusion can accumulate large amount of reads, which leads to significant increase in peak memory utilization. Imagine we have the following code snippet
```
total = torch.rand(N, N)
for _ in range(r):
x = torch.rand(N, N)
total = total + x
```
The default execution is memory efficient as only two tensors of size N-by-N is in memory at any given time. However, with fusion, the additions are fused into a single operation and the execution becomes something like:
```
x_1 = torch.rand(N, N)
x_2 = torch.rand(N, N)
...
x_r = torch.rand(N, N)
total = x_1 + x_2 + ... + x_r
```
Though this is run-time efficient, in the case of large `N` and/or large `r`, this is not memory efficient.
[internal only] see [post](https://fb.workplace.com/groups/1075192433118967/permalink/1703374333634104/) for additional details
**Solution:**
Our proposed solution is to ban fusions in case where a large amount of reads are accumulated. This is in addition to some existing logics during torch compile.
* During lowering (i.e., `ir.py`), the config `realize_acc_reads_threshold`, which is default to be 8, controls _the number of_ buffers can be accumulated for a single operator. However, this is oblivious to the size of the buffers. Hence, we additionally introduce a config `realize_acc_reads_size_threshold` to control _the amount of buffers_ in size that can be accumulated.
* During scheduling (i.e., `scheduler.py`), additional fusion will be performed and thus we also need to capture such pattern there. The decisions are implemented under `choices.py`.
**Results:**
For a small example similar to be one in the test case (but with larger `N` and higher number of loop repeats), the memory snapshot before and after are shown below. Note the snapshot on the right is zoomed out so that the y-axis of the two snapshots match.
<img width="1328" alt="image" src="https://github.com/user-attachments/assets/670b5961-8454-4379-ae0f-62d4e7946c64" />
Pull Request resolved: https://github.com/pytorch/pytorch/pull/157563
Approved by: https://github.com/jansel, https://github.com/mlazos
**Problem:**
Fusion can accumulate large amount of reads, which leads to significant increase in peak memory utilization. Imagine we have the following code snippet
```
total = torch.rand(N, N)
for _ in range(r):
x = torch.rand(N, N)
total = total + x
```
The default execution is memory efficient as only two tensors of size N-by-N is in memory at any given time. However, with fusion, the additions are fused into a single operation and the execution becomes something like:
```
x_1 = torch.rand(N, N)
x_2 = torch.rand(N, N)
...
x_r = torch.rand(N, N)
total = x_1 + x_2 + ... + x_r
```
Though this is run-time efficient, in the case of large `N` and/or large `r`, this is not memory efficient.
[internal only] see [post](https://fb.workplace.com/groups/1075192433118967/permalink/1703374333634104/) for additional details
**Solution:**
Our proposed solution is to ban fusions in case where a large amount of reads are accumulated. This is in addition to some existing logics during torch compile.
* During lowering (i.e., `ir.py`), the config `realize_acc_reads_threshold`, which is default to be 8, controls _the number of_ buffers can be accumulated for a single operator. However, this is oblivious to the size of the buffers. Hence, we additionally introduce a config `realize_acc_reads_size_threshold` to control _the amount of buffers_ in size that can be accumulated.
* During scheduling (i.e., `scheduler.py`), additional fusion will be performed and thus we also need to capture such pattern there. The decisions are implemented under `choices.py`.
**Results:**
For a small example similar to be one in the test case (but with larger `N` and higher number of loop repeats), the memory snapshot before and after are shown below. Note the snapshot on the right is zoomed out so that the y-axis of the two snapshots match.
<img width="1328" alt="image" src="https://github.com/user-attachments/assets/670b5961-8454-4379-ae0f-62d4e7946c64" />
Pull Request resolved: https://github.com/pytorch/pytorch/pull/157563
Approved by: https://github.com/jansel, https://github.com/mlazos
Softmax need do some preparation work that access the input tensor in two passes
- compute amax of each row
- compute (x - amax).exp.sum for each row
When the row size is large, cache can not hold all the active data and accessing the input multiple passes increases execution time since the kernel is membw bounded.
Online softmax uses a customized reduction to compute max and sum at the same time by accessing the data in one pass. Check this paper for more details ( https://arxiv.org/abs/1805.02867 ).
Also here is an online softmax kernel generated by inductor as a reference: https://gist.github.com/shunting314/67ae4fffd45d4f2753c781780332fa54
## Microbenchmark
- `TORCHINDUCTOR_COORDINATE_DESCENT_TUNING=1 TORCHINDUCTOR_ONLINE_SOFTMAX=0 DO_PERF_TEST=1 python test/inductor/test_online_softmax.py -k test_softmax` : without online softmax
- eager_ms=6.671296119689941
- opt_ms=8.06931209564209
- `TORCHINDUCTOR_COORDINATE_DESCENT_TUNING=1 TORCHINDUCTOR_ONLINE_SOFTMAX=1 DO_PERF_TEST=1 python test/inductor/test_online_softmax.py -k test_softmax`: with online softmax
- eager_ms=6.634047985076904
- opt_ms=6.230591773986816
Ideally, online softmax should save about 2ms here. We saves about 1.84ms in practice.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/127011
Approved by: https://github.com/jansel
