Perfetto trace loader doesn't support "FEXTRA" multi-block gzip files. How to reproduce:

1. Install https://github.com/vinlyx/mgzip
2. Take any exiting JSON trace
3. Code

with open("trace.json") as fin:
    with mgzip.open("trace.json.gz", "wt", thread=8, blocksize=1 << 16) as fout:
        while buffer := fin.read(1 << 16):
            fout.write(buffer)

4. For example,

../trace_processor --httpd trace.json.gz
JSON trace file is incomplete

5. This will work:

gzip -d trace.json.gz
gzip trace.json
../trace_processor --httpd trace.json.gz

Why does this weird gzip format property matter to me? We, 100B-parameter base LLM trainers in PyTorch, deal with a few hundred megs of profile that require considerable time to compress every few minutes, so compressing them on 192 available CPU cores gives a considerable benefit.