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Re: Fastest way to run two external processes

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On Wed, 29 Apr 2026 07:38:23 -0000 (UTC), Mark Summerfield wrote:

> I need to run two external processes (on Linux):
> 
> pdftotext -tsv one.pdf
> pdftotext -tsv two.pdf
> 
> For each one I need to acquire the output and post-process it.
> Both are completely independent.
> (However, once I've finished post-processing I then do some work on
> both sets of post-processed data together.)
> 
> Each external process takes about 3 secs so it takes just over 6 secs
> to acquire the data from both processes.
> 
> When I've done something similar in Python I've used the multiprocessing
> module and this has got my runtime close to the 3 secs.
> 
> In my experiments with Tcl's threading I've found the threading startup
> overhead to be rather large.
> 
> What is the fastest way to run two independent processes concurrently
> and acquire their outputs using Tcl?

Here's my serial version:

proc app::serial {pdftotext pdf1 pdf2} {
    puts serial
    set pdf1tsv [exec $pdftotext -tsv $pdf1 -]
    set pdf2tsv [exec $pdftotext -tsv $pdf2 -]
    list $pdf1tsv $pdf2tsv
}

This takes ~2 sec for two ~650 page PDFs.

With some help from Gemini (after I got past non-working and slow
solutions) I did a multiprocessing version:

proc app::multiprocess {pdftotext pdf1 pdf2} {
    set p1 [open "|$pdftotext -tsv $pdf1 - 2>@1" r]
    try {
        set p2 [open "|$pdftotext -tsv $pdf2 - 2>@1" r]
        try {
            fconfigure $p1 -blocking 0
            fconfigure $p2 -blocking 0
            set pdf1tsv ""
            set pdf2tsv ""
            while {![eof $p1] || ![eof $p2]} {
                append pdf1tsv [read $p1]
                append pdf2tsv [read $p2]
                after 1
            }
        } finally {
            close $p2
        }
    } finally {
        close $p1
    }
    list $pdf1tsv $pdf2tsv
}

This takes ~1 sec.

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Thread

Fastest way to run two external processes Mark Summerfield <m.n.summerfield@gmail.com> - 2026-04-29 07:38 +0000
  Re: Fastest way to run two external processes Mark Summerfield <m.n.summerfield@gmail.com> - 2026-04-29 08:51 +0000
    Re: Fastest way to run two external processes meshparts <alexandru.dadalau@meshparts.de> - 2026-04-29 11:24 +0200
      Re: Fastest way to run two external processes Mark Summerfield <m.n.summerfield@gmail.com> - 2026-04-29 09:46 +0000
    Re: Fastest way to run two external processes Ralf Fassel <ralfixx@gmx.de> - 2026-04-29 12:30 +0200
      Re: Fastest way to run two external processes abu <user13892@newsgrouper.org.invalid> - 2026-04-30 00:51 +0000
        Re: Fastest way to run two external processes Ralf Fassel <ralfixx@gmx.de> - 2026-04-30 14:23 +0200
        Re: Fastest way to run two external processes Mark Summerfield <m.n.summerfield@gmail.com> - 2026-05-01 07:04 +0000
          Re: Fastest way to run two external processes Ralf Fassel <ralfixx@gmx.de> - 2026-05-01 22:54 +0200
            Re: Fastest way to run two external processes Mark Summerfield <m.n.summerfield@gmail.com> - 2026-05-02 06:57 +0000
              Re: Fastest way to run two external processes Ashok <apnmbx-public@yahoo.com> - 2026-05-09 16:03 +0530
                Re: Fastest way to run two external processes Ralf Fassel <ralfixx@gmx.de> - 2026-05-11 11:08 +0200
                Re: Fastest way to run two external processes Mark Summerfield <m.n.summerfield@gmail.com> - 2026-05-12 08:40 +0000
                Re: Fastest way to run two external processes Ralf Fassel <ralfixx@gmx.de> - 2026-05-12 16:58 +0200
  Re: Fastest way to run two external processes Olivier <user1108@newsgrouper.org.invalid> - 2026-05-01 10:06 +0000
  Re: Fastest way to run two external processes Emiliano <emiliano@example.invalid> - 2026-05-02 00:34 -0300

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