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151 lines
6.6 KiB
ReStructuredText
151 lines
6.6 KiB
ReStructuredText
Performance
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===================
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The main goals for |kitty| performance are user perceived latency while typing
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and "smoothness" while scrolling as well as CPU usage. |kitty| tries hard to
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find an optimum balance for these. To that end it keeps a cache of each
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rendered glyph in video RAM so that font rendering is not a bottleneck.
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Interaction with child programs takes place in a separate thread from
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rendering, to improve smoothness. Parsing of the byte stream is done using
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`vector CPU instructions
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<https://en.wikipedia.org/wiki/Single_instruction,_multiple_data>`__ for
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maximum performance. Updates to the screen typically require sending just a few
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bytes to the GPU.
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There are two config options you can tune to adjust the performance,
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:opt:`repaint_delay` and :opt:`input_delay`. These control the artificial delays
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introduced into the render loop to reduce CPU usage. See
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:ref:`conf-kitty-performance` for details. See also the :opt:`sync_to_monitor`
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option to further decrease latency at the cost of some `screen tearing
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<https://en.wikipedia.org/wiki/Screen_tearing>`__ while scrolling.
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Benchmarks
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-------------
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Measuring terminal emulator performance is fairly subtle, there are three main
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axes on which performance is measured: Energy usage for typical tasks,
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Keyboard to screen latency, and throughput (processing large amounts of data).
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Keyboard to screen latency
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This is measured either with dedicated hardware, or software such as `Typometer
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<https://pavelfatin.com/typometer/>`__. Third party measurements comparing
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kitty with other terminal emulators on various systems show kitty has best in
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class keyboard to screen latency.
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Note that to minimize latency at the expense of more energy usage, use the
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following settings in kitty.conf::
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input_delay 0
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repaint_delay 2
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sync_to_monitor no
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wayland_enable_ime no
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`Hardware based measurement on macOS
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<https://thume.ca/2020/05/20/making-a-latency-tester/>`__ show that kitty and
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Apple's Terminal.app share the crown for best latency. These
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measurements were done with :opt:`input_delay` at its default value of ``3 ms``
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which means kitty's actual numbers would be even lower.
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`Typometer based measurements on Linux
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<https://github.com/kovidgoyal/kitty/issues/2701#issuecomment-911089374>`__
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show that kitty has far and away the best latency of the terminals tested.
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.. _throughput:
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Throughput
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^^^^^^^^^^^^^^^^
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kitty has a builtin kitten to measure throughput, it works by dumping large
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amounts of data of different types into the tty device and measuring how fast
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the terminal parses and responds to it. The measurements below were taken with
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the same font, font size and window size for all terminals, and default
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settings, on the same computer. They clearly show kitty has the fastest
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throughput. To run the tests yourself, run ``kitten __benchmark__`` in the
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terminal emulator you want to test, where the kitten binary is part of the
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kitty install.
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The numbers are megabytes per second of data that the terminal
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processes. Measurements were taken under Linux/X11 with an ``AMD Ryzen 7 PRO
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5850U``. Entries are in order of decreasing performance. kitty is twice
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as fast as the next best.
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================ ====== ======= ===== ====== =======
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Terminal ASCII Unicode CSI Images Average
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================ ====== ======= ===== ====== =======
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kitty 0.33 121.8 105.0 59.8 251.6 134.55
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gnometerm 3.50.1 33.4 55.0 16.1 142.8 61.83
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alacritty 0.13.1 43.1 46.5 32.5 94.1 54.05
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wezterm 20230712 16.4 26.0 11.1 140.5 48.5
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xterm 389 47.7 18.3 0.6 56.3 30.72
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konsole 23.08.04 25.2 37.7 23.6 23.4 27.48
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alacritty+tmux 30.3 7.8 14.7 46.1 24.73
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================ ====== ======= ===== ====== =======
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In this table, each column represents different types of data. The CSI column
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is for data consisting of a mix of typical formatting escape codes and some
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ASCII only text.
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.. note::
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By default, the benchmark kitten suppresses actual rendering, to better
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focus on parser speed, you can pass it the ``--render`` flag to not suppress
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rendering. However, modern terminals typically render asynchronously,
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therefore the numbers are not really useful for comparison, as it is just a
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game about how much input to *batch* before rendering the next frame.
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However, even with rendering enabled kitty is still faster than all the
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rest. For brevity those numbers are not included.
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.. note::
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foot, iterm2 and Terminal.app are left out as they do not run under X11.
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Alacritty+tmux is included just to show the effect of putting a terminal
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multiplexer into the mix (halving throughput) and because alacritty isnt
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remotely comparable to any of the other terminals feature wise without tmux.
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.. note::
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konsole, gnome-terminal and xterm do not support the `Synchronized update
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<https://gitlab.com/gnachman/iterm2/-/wikis/synchronized-updates-spec>`__
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escape code used to suppress rendering, if and when they gain support for it
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their numbers are likely to improve by ``20 - 50%``, depending on how well they
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implement it.
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Energy usage
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^^^^^^^^^^^^^^^^^
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Sadly, I do not have the infrastructure to measure actual energy usage so CPU
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usage will have to stand in for it. Here are some CPU usage numbers for the
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task of scrolling a file continuously in :program:`less`. The CPU usage is for
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the terminal process and X together and is measured using :program:`htop`. The
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measurements are taken at the same font and window size for all terminals on a
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``Intel(R) Core(TM) i7-4820K CPU @ 3.70GHz`` CPU with a ``Advanced Micro
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Devices, Inc. [AMD/ATI] Cape Verde XT [Radeon HD 7770/8760 / R7 250X]`` GPU.
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============== =========================
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Terminal CPU usage (X + terminal)
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============== =========================
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|kitty| 6 - 8%
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xterm 5 - 7% (but scrolling was extremely janky)
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termite 10 - 13%
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urxvt 12 - 14%
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gnome-terminal 15 - 17%
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konsole 29 - 31%
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============== =========================
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As you can see, |kitty| uses much less CPU than all terminals, except xterm, but
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its scrolling "smoothness" is much better than that of xterm (at least to my,
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admittedly biased, eyes).
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Instrumenting kitty
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-----------------------
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You can generate detailed per-function performance data using
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`gperftools <https://github.com/gperftools/gperftools>`__. Build |kitty| with
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``make profile``. Run kitty and perform the task you want to analyse, for
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example, scrolling a large file with :program:`less`. After you quit, function
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call statistics will be displayed in *KCachegrind*. Hence, profiling is best done
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on Linux which has these tools easily available.
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