Monday, July 5, 2021

Looking at the performance of Refterm

Recently a known strong-opinion-holder Casey Muratori wrote a reference implementation for a new fast terminal renderer.  The goal is (as far as I can tell) to implement the same functionality as existing terminals using a lot less resources. The code is here and according to the readme it supports, for example:

  • All of Unicode, including combining characters and right-to-left text like Arabic
  • Line wrapping
  • Reflowing line wrapping on terminal resize
The tests they perform show that the new terminal works several orders of magnitude faster than the default terminals in Windows. Seems nice, so let's do a simple code review to see how it actually stacks up.

Code setup

The code is arranged in a very non-standard way. There are two main source files, one C and one C++ that are built with a BAT file. Those files first #define a bunch of stuff and then #include all other source code files (not header files, source files) directly. This is very old school and against pretty much any recommended best practice for C. This works will for single-person projects but is a big hurdle for any new contributors.

The build uses the /GS- /Gs999999set command line arguments, which disable security features. This seems ill-advised for a terminal application, whose main job is to parse untrusted input. All libraries used are not defined in the build file but instead as pragmas inside the source files. The program also does not link the C library and because of this has its own simple implementations of memcpy and memset. This means you don't get the SIMD-optimized versions from the stdlib (the performance impact of this was not tested).

Resource usage

Resource consumption was measured by checking out the code, building it with the bat as instructed by upstream, starting the program and letting it idle. This is how it appears in Windows' task manager.

The app uses 0.5% of CPU and a whopping 14% of GPU just to display a blinking cursor. This could be said to be not particularly resource efficient. This is probably due to the fact that there is no rate limiter (or VSYNC) so the app just spams the system all the time. The true resource usage can't be meaningfully compared until this is fixed.

What can be measured, though, is memory usage. As can be seen in the image [1] the Refterm application uses 351 MB of memory when idle (the test war run using a 4k monitor). Based on discussions on the Internet, an acceptable amount of memory usage for a terminal is around 10-20 MB. Refterm uses 10x as much. In fact, as you can tell, running two instances of Refterm takes more memory than a fully blown Firefox with tens of open tabs. For comparison I also tested the Spotify app which is implemented in Electron. When playing music it only took ~150 MB, less than half of an idling Refterm.


A terminal widget is a high risk app because it has to parse a lot of untrusted text. Thus it is very important that it tolerates malformed and malicious input. A simple test is to start Refterm and then run splat refterm_debug_msvc.pdb in Refterm. This basically dumps 1.3 MB of binary data to the terminal. This causes Refterm to immediately freeze and take 100% CPU. The app window can not be closed and can only be killed via the task manager.


In its current form Refterm can, at most, be seen as a proof of concept which can not be reasonably compared against a full-featured terminal renderer. It is neither memory-efficient nor reliable against malformed input.

[1] I don't know much about Windows development, so I don't know how representative this number is of the "real" resource usage. At least on Linux the output of top should be taken with a grain of salt. I also tried VS's profiler and it claimed that the app took over 450 MB of ram.

Update 2021/7/7

This blog post got linked to from places which caused a flood of mostly nonproductive and toxic comments. Because of this no further comments are allowed for this post. Sorry.

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