Performance
===================

The main goals for |kitty| performance are user perceived latency while typing
and "smoothness" while scrolling as well as CPU usage. |kitty| tries hard to find
an optimum balance for these. To that end it keeps a cache of each rendered
glyph in video RAM so that font rendering is not a bottleneck.  Interaction
with child programs takes place in a separate thread from rendering, to improve
smoothness.

There are two parameters you can tune to adjust the performance. :opt:`repaint_delay`
and :opt:`input_delay`. These control the artificial delays introduced into the
render loop to reduce CPU usage. See :ref:`conf-kitty-performance` for details.
See also the :opt:`sync_to_monitor` option to further decrease latency at the cost
of some `tearing <https://en.wikipedia.org/wiki/Screen_tearing>`_ while scrolling.

You can generate detailed per-function performance data using `gperftools
<https://github.com/gperftools/gperftools>`_. Build |kitty| with `make
profile`. Run kitty and perform the task you want to analyse, for example,
scrolling a large file with `less`. After you quit, function call statistics
will be printed to `stdout` and you can use tools like *kcachegrind* for more
detailed analysis.

Here are some CPU usage numbers for the task of scrolling a file continuously
in less.  The CPU usage is for the terminal process and X together and is
measured using htop.  The measurements are taken at the same font and window
size for all terminals on a ``Intel(R) Core(TM) i7-4820K CPU @ 3.70GHz`` CPU
with a ``Advanced Micro Devices, Inc. [AMD/ATI] Cape Verde XT [Radeon HD
7770/8760 / R7 250X]`` GPU.

==============   =========================
Terminal         CPU usage (X + terminal)
==============   =========================
|kitty|          6 - 8%
xterm            5 - 7% (but scrolling was extremely janky)
termite          10 - 13%
urxvt            12 - 14%
gnome-terminal   15 - 17%
konsole          29 - 31%
==============   =========================


As you can see, |kitty| uses much less CPU than all terminals, except xterm, but
its scrolling "smoothness" is much better than that of xterm (at least to my,
admittedly biased, eyes).


.. _perf-cat:

.. note::

    Some people have asked why kitty does not perform better than terminal XXX in
    the test of sinking large amounts of data, such as catting a large text
    file. The answer is because this is not a goal for kitty. kitty
    deliberately throttles input parsing and output rendering to minimize
    resource usage while still being able to sink output faster than any real
    world program can produce it. Reducing CPU usage, and hence battery drain
    while achieving instant response times and smooth scrolling to a human eye
    is a far more important goal.