What about qpdf multithreaded?

[Phil353556]

Hello,

First thanks to all who make it happen. I mainly use stirling PDF on a pi5 under docker, the goal is to compress pdf file.

I notice that the program ran to compress is qpdf, and qpdf is a mono thread program.
I don't know if there is a way to modify it in a multithread program?

CPU activity monitored with the nmon program.

Thanks for your time and answer
Phil

[dosubot[bot]]

Hi @Phil353556! I'm Dosu and I'm helping the Stirling-tools team.

You're correct that QPDF itself is a single-threaded program — there's no built-in way to make a single QPDF invocation use multiple cores. However, Stirling-PDF's compression pipeline is a bit more involved than just QPDF alone:

How compression actually works in Stirling-PDF:

• At optimization levels 1–5, only QPDF is used for structural/stream compression [1].
• At optimization levels 6+, both Ghostscript and QPDF are applied sequentially — Ghostscript handles aggressive image downsampling/deduplication, then QPDF handles stream recompression [1].

Regarding multithreading:

• Within a single PDF compression job, the tools run sequentially — there's no parallelization for one file [1].
• At the request level, Stirling-PDF allows multiple concurrent compression sessions (default: 8 for Ghostscript, 2 for QPDF), so if you're processing multiple PDFs at once, they can run in parallel [2].

Practical suggestions for your Pi 5:

• If you're compressing one large file at a time, you'll unfortunately see only one core utilized — this is a limitation of QPDF and Ghostscript themselves, not something Stirling-PDF can easily work around.
• If you have multiple files to compress, submitting them concurrently (or using batch processing) would leverage more cores.
• Using a higher optimization level (6+) engages Ghostscript as well, which may yield better compression ratios even if it doesn't parallelize within a single file.

There isn't currently a way to make QPDF itself multithreaded — that would require upstream changes to the QPDF project. This is a known architectural characteristic of QPDF rather than something configurable in Stirling-PDF.

To reply, just mention @dosu.

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[Phil353556]

Hello @dosu,

Thanks for your complete explanation. I appreciate

Phil

[dosubot[bot]]

You're welcome, @Phil353556! Glad it was helpful. Feel free to ask if you have any other questions down the road. Happy compressing! 🙂

To reply, just mention @dosu.

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Docs are dead. Just use Dosu.