Question on pseudo console and legacy console

[Koichi Murase]

I have a question on the pseudo console mode and the legacy console
mode.

Question:

  Does it cause problems to set `disable_pcon' in the default value of
  the environment variable `CYGWIN'?  When is the pseudo console mode
  recommended, and when can I use the legacy console mode?  I noticed
  that the terminal emulators with the pseudo console mode is quite
  slower than with the legacy console mode, so I am thinking of
  setting `disable_pcon' by default if it does not cause problem.


Background:

* There are large performance differences of terminal emulators
  between the legacy and pseudo console modes.  I show some test and
  results in my laptop as follows:

  For example, one can measure a performance of the terminal emulators
  by the following commands.  It measures the processing time of the
  terminal for 1M lines.

  $ yes | head -1000000 > yes.txt
  $ time cat yes.txt

  The results with different terminals and different console modes
  are summarized below:

  urxvt     ... legacy: 0.410s,   pcon: 33.517s   (~ 80x slower)
  (A) win   ... legacy: 1.593s,   pcon: 36.064s   (~ 20x slower)
  (B) x11   ... legacy: 1.573s,   pcon: 35.611s   (~ 20x slower)
  (C) tty   ... legacy: 1.614s,   pcon: 35.129s   (~ 20x slower)
  mintty    ... legacy: 3.811s,   pcon: 39.406s   (~ 10x slower)
  xterm -j  ... legacy: 39.809s,  pcon: 1m2.237s  (~ 1.5x slower)

  [ Note: `legacy' and `pcon' are the legacy and pseudo console
  modes, respectively.  (A)--(C) are three different modes of the
  terminal that I wrote by myself and also I primarily use daily.
  When xterm is tested, the option -j is passed to explicitly turn
  off the smooth scrolling mode. ]

  This is the results for a specific computer, but the difference
  between legacy and pseudo console modes is clear.  I think the
  reason why pseudo console is so slow is that it actually
  internally processes terminal sequences and constructs its
  terminal contents in backgrounds, which is completely redundant
  process when the user uses terminal emulators.  And the background
  terminal is as slow as xterm, which is at least ten times slower
  than the other terminal emulators.

* I understand that, with the pseudo console mode, the Cygwin console
  have more powerful terminal features compared to the legacy console
  mode.  However, I do not usually use the console but another
  terminal emulator.  Also the default Cygwin Terminal (which can be
  launched from the shortcut icon on desktop or in the start menu) is
  actually Mintty, so I believe most other people also use terminal
  emulators.

  Is there any reason to enable the pseudo console mode for all the
  programs including those which does not use the console window?  For
  example, does the Cygwin PTY with the legacy console mode have some
  limitation which is not present with the pseudo console mode?  Or,
  are there some problems caused by the legacy console mode?  I
  thought maybe some Windows Command-Line application (based on
  Windows Console API) could have troubles with the legacy console
  mode, but with a quick check for `cmd' and `PowerShell' it appears
  to work in the terminal emulators with the legacy console mode
  though I haven't tested it thoroughly.

* For these reasons, if there is no functional differences between the
  pseudo console mode and the legacy console mode as far as I use
  terminal emulators, I tend to think about setting `disable_pcon' as
  the default value of the environment variable `CYGWIN'.  But looking
  at the discussion at

  https://cygwin.com/ml/cygwin-patches/2020-q1/threads.html#00060

  it appears the setting `disable_pcon' is only introduced as a
  workaround for the programs incompatible with the pseudo console
  mode (such as `cgdb'), and the pseudo console mode is still
  considered to be appropriate for normal programs.

* If the console mode would not cause any differences when there is no
  visible console window, would it make sense to change Cygwin's
  behavior so that it switches to the pseudo console mode only when
  the console window is present (if it is not too difficult
  technically)?


Best regards,

Koichi

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