|kitty| 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. ``repaint_delay``
and ``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 ``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` which will create an executable called `kitty-profile`.  Run
that 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).