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lpereira / lwan


Experimental, scalable, high performance HTTP server


lwan Web Server

Lwan is a high-performance & scalable web server for glibc/Linux platforms.

In development for almost 3 years, Lwan was until now a personal research effort that focused mostly on building a solid infrastructure for a lightweight and speedy web server:

  • Low memory footprint (~500KiB for 10k idle connections)
  • Minimal memory allocations & copies
  • Minimal system calls
  • Hand-crafted HTTP request parser
  • Files are served using the most efficient way according to their size
    • No copies between kernel and userland for files larger than 16KiB
    • Smaller files are sent using vectored I/O of memory-mapped buffers
    • Header overhead is considered before compressing small files
  • Mostly wait-free multi-threaded design
  • Diminute code base with roughly 7200 lines of C code

It is now transitioning into a fully working, capable HTTP server. It is not, however, as feature-packed as other popular web servers. But it is free software, so scratching your own itches and making Lwan hum the way you want it to is possible.

Features include:

  • Mustache templating engine
    • Used for directory listing & error messages
    • Available for user-built handlers
  • Easy to use API to create web applications or extend the web server
  • Supports rebimboca da parafuseta
  • Test suite written in Python tests the server as a black box
  • No-nonsense configuration file syntax
  • Supports a subset of HTTP/1.0 and HTTP/1.1
  • systemd socket activation
  • IPv6 ready

The web site has more details, including a FAQ about the name of the project and security concerns.


It can achieve good performance, yielding about 320000 requests/second on a Core i7 laptop for requests without disk access.

When disk I/O is required, for files up to 16KiB, it yields about 290000 requests/second; for larger files, this drops to 185000 requests/second, which isn't too shabby either.

These results, of course, with keep-alive connections, and with weighttp running on the same machine (and thus using resources that could be used for the webserver itself).

Without keep-alive, these numbers drop around 6-fold.


Although it uses epoll and the Linux variant of sendfile(), it is fairly portable to other event-based pollers, like kqueue. An old version of lwan has been successfully ported to FreeBSD. Eventually, some event library such as libev or libevent will be used to aid in portability.


Before installing Lwan, ensure all dependencies are installed. All of them are common dependencies found in any GNU/Linux distribution; package names will be different, but it shouldn't be difficult to search using whatever package management tool that's used by your distribution.

Required dependencies

  • CMake, at least version 2.8
  • Python, at least version 2.6 (3.x is OK)
  • ZLib

Optional dependencies

The build system will look for these libraries and enable/link if available.

Common distribution package names

  • ArchLinux: pacman -S cmake python zlib sqlite luajit libmariadbclient gperftools valgrind
  • Ubuntu 14: apt-get update && apt-get install git cmake zlib1g-dev pkg-config lua5.1-dev libsqlite3-dev libmysqlclient-dev

Build commands

~$ git clone git://
~$ cd lwan
~/lwan$ mkdir build
~/lwan$ cd build
~/lwan/build$ cmake .. -DCMAKE_BUILD_TYPE=Release
~/lwan/build$ make

It is important to build outside of the tree; in-tree builds are not supported.

Passing -DCMAKE_BUILD_TYPE=Release will enable some compiler optimizations (such as LTO) and tune the code for current architecture. Please use this version when benchmarking, as the default is the Debug build, which not only logs all requests to the standard output, but does so while holding a mutex.

The default build (i.e. not passing -DCMAKE_BUILD_TYPE=Release) will build a version suitable for debugging purposes. This version can be used under Valgrind, is built with Undefined Behavior Sanitizer, and includes debugging messages that are stripped in the release version. Debugging messages are printed while holding a mutex, and are printed for each and every request; so do not use this version for benchmarking purposes.


Set up the server by editing the provided lwan.conf; the format is very simple and should be self-explanatory.

Configuration files are loaded from the current directory. If no changes are made to this file, running lwan will serve static files located in the ./wwwroot directory, and also provide a Hello, World! handler (which serves as an example of how to use some of its internal APIs).

Lwan will listen on port 8080 on all interfaces.

Lwan will detect the number of CPUs, will increase the maximum number of open file descriptors and generally try its best to autodetect reasonable settings for the environment it's running on.

Optionally, the lwan binary can be used for one-shot static file serving without any configuration file. Run it with --help for help on that.

IRC Channel

There is an IRC channel (#lwan) on Freenode. A standard IRC client can be used. A web IRC gateway is also available.

Lwan in the wild

Here's a non-definitive list of third-party stuff that uses Lwan and have been seen in the wild. Help build this list!

Some other distribution channels were made available as well:

Lwan has been also used as a benchmark:

Not really third-party, but alas:

Build status

Release Debug Static Analysis Unit Tests
release debug clang coverity tests
Waterfall Waterfall Waterfall - Reports Waterfall