`compile_and_save` needs twice the engine size on disk while it saves to ExecuTorch

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
18/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
python, pytorch
Domain
data, devtools

Research direction

The save path is in compile_and_save in the Torch-TensorRT Python package, which writes each built engine to a temporary file before the ExecuTorch copy; the repro script in the issue measures peak disk use and is the check for done. The fix spans two projects: the ExecuTorch file-writing code that would need to punch holes in owned file-backed data after each chunk is copied, and the Torch-TensorRT hand-off of offloaded engine files. Start by reading how engine files are written and passed to ExecuTorch, then confirm the disk peak drops to about one copy.

Written by the indexing model from the issue text.

Description

What is wrong

compile_and_save writes each engine to a temporary file as soon as it is built. With output_format="executorch", the save then copies each file into the .pte (or .ptd). The temporary files are deleted only when the call returns. So at the end of the save, every engine is on disk twice.

For a large single-engine program this decides whether the export fits. #5026 lets the caller move the files to another disk, which helps, but the space is still needed.

What this blocks

No policy in pytorch/executorch#24353 is listed as failing because of this. It costs disk.
Any export with output_format="executorch" needs free space for twice its engines, so a large single-engine program needs twice its size free. That can decide whether it fits on a Jetson or a DGX Spark.
A fix lets an export fit when the free disk is only a little larger than the program.

Repro
import os, shutil, tempfile, threading, time
import torch, torch_tensorrt

work = tempfile.mkdtemp()
os.environ["TMPDIR"] = tempfile.tempdir = os.path.join(work, "tmp")
os.makedirs(tempfile.tempdir)
out = os.path.join(work, "model.pte")

def used(path):
    return sum(os.stat(os.path.join(r, f)).st_blocks * 512 for r, _, fs in os.walk(path) for f in fs)

peak, stop = [0], threading.Event()
def watch():
    while not stop.is_set():
        peak[0] = max(peak[0], used(work))
        time.sleep(0.2)

model = torch.nn.Sequential(*[torch.nn.Linear(8192, 8192, bias=False) for _ in range(32)]).cuda().bfloat16().eval()
x = torch.randn(4, 8192, device="cuda", dtype=torch.bfloat16)
ep = torch.export.export(model, (x,))
threading.Thread(target=watch, daemon=True).start()
torch_tensorrt.compile_and_save(ep, out, output_format="executorch", arg_inputs=[x], min_block_size=1)
stop.set()
print(f"program {os.path.getsize(out) / 2**30:.2f} GiB, disk peak {peak[0] / 2**30:.2f} GiB")
shutil.rmtree(work)
Actual
program 4.00 GiB, disk peak 8.00 GiB
Expected

The peak is about one copy. A proposal in 2 small parts:

  1. ExecuTorch: file-backed data that the caller marks as owned is consumed by the write. After each chunk is copied, that range of the source is released (fallocate with FALLOC_FL_PUNCH_HOLE, supported on ext4, XFS and btrfs), and the file is removed at the end. Elsewhere it copies as today.
  2. Torch-TensorRT: hand each offloaded engine file to ExecuTorch as owned data, since nothing reads it after the save.
Versions

torch 2.15.0 (CUDA 13.2), torch_tensorrt 2.15.0, executorch 1.6.0, TensorRT 11.4.0.106. Script run on H100, driver 580.126.09 (btrfs).

Related: #4863 (keeping built engines on disk until the save, which is where the temporary files come from), #5026.

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