Event Loop Implementations: libuv and libev#

New in version 1.3.

gevent offers a choice of two event loop libraries (libev and libuv) and three event loop implementations. This document will explore those implementations and compare them to each other.

Using A Non-Default Loop#

First, we will describe how to choose an event loop other than the default loop for a given platform. This is done by setting the GEVENT_LOOP environment variable before starting the program, or by setting gevent.config.loop in code.


If you choose to configure the loop in Python code, it must be done immediately after importing gevent and before any other gevent imports or asynchronous operations are done, preferably at the top of your program, right above monkey-patching (if done):

import gevent
gevent.config.loop = "libuv"


In gevent 1.4 and 1.3, if you install gevent from a manylinux1 binary wheel as distributed on PyPI, you will not be able to use the libuv loop. You’ll need to compile from source to gain access to libuv. gevent 1.5 distributes manylinux2010 wheels which have libuv support.

If you use a Linux distribution’s package of gevent, you may or may not have any other loops besides the default.

Loop Implementations#

Here we will describe the available loop implementations.







Build Status




CPython on non-Windows platforms

CPython only

8 years



Default; optional



PyPy on non-Windows platforms

CPython and PyPy

4 years


Optional; default

Default; optional



All interpreters on Windows

CPython and PyPy

2 years


Optional; default

Default; optional


libev is a venerable event loop library that has been the default in gevent since 1.0a1 in 2011 when it replaced libevent. libev has existed since 2007.


In the future, this Cython implementation may be deprecated to be replaced with libev-cffi.

Development and Source

libev is a stable library and does not change quickly. Changes are accepted in the form of patches emailed to a mailing list. Due to its age and its portability requirements, it makes heavy use of preprocessor macros, which some may find hinders readability of the source code.

Platform Support

gevent tests libev on Linux and macOS. There is no known list of platforms officially supported by libev, although FreeBSD, OpenBSD and Solaris/SmartOS have been reported to work with gevent on libev at various points.

On Windows, libev has many limitations. gevent relies on the Microsoft C runtime functions to map from Windows socket handles to integer file descriptors for libev using a somewhat complex mapping; this prevents the CFFI implementation from being used (which in turn prevents PyPy from using libev on Windows).

There is no known public CI infrastructure for libev itself.


This uses libev exactly as above, but instead of using Cython it uses CFFI. That makes it suitable (and the default) for PyPy. It can also make it easier to debug, since more details are preserved for tracebacks.


In the future, this CFFI implementation may become the default and replace libev.

When To Use

On PyPy or when debugging.


libuv is an event loop library developed since 2011 for the use of node 0.5. It was originally a wrapper around libev on non-Windows platforms and directly used the native Windows IOCP support on Windows (this code was contributed by Microsoft). Now it has its own loop implementation on all supported platforms.

libuv provides libev-like “poll handles”, and in gevent 1.3 that is what gevent uses for IO. But libuv also provides higher-level abstractions around read and write requests that may offer improved performance. In the future, gevent might use those abstractions.


In the future, this implementation may become the default on all platforms.

Development and Source

libuv is developed by the libuv organization on github. It has a large, active community and is used in many popular projects including node.js.

The source code is written in a clean and consistent coding style, potentially making it easier to read and debug.

Platform Support

gevent tests libuv on Linux, Windows and macOS. libuv publishes an extensive list of supported platforms that are likely to work with gevent. libuv maintains a public CI infrastructure.

When To Use libuv

  • You want to use PyPy on Windows.

  • You want to develop on Windows (Windows is not recommended for production).

  • You want to use an operating system not supported by libev such as IBM i.


    Platforms other than Linux, macOS and Windows are not tested by gevent.

Limitations and Differences#

Because of its newness, and because of some design decisions inherent in the library and the ecosystem, there are some limitations and differences in the way gevent behaves using libuv compared to libev.

  • Timers (such as gevent.sleep and gevent.Timeout) only support a resolution of 1ms (in practice, it’s closer to 1.5ms). Attempting to use something smaller will automatically increase it to 1ms and issue a warning. Because libuv only supports millisecond resolution by rounding a higher-precision clock to an integer number of milliseconds, timers apparently suffer from more jitter.

  • Using negative timeouts may behave differently from libev.

  • libuv blocks delivery of all signals, so signals are handled using an (arbitrary) 0.3 second timer. This means that signal handling will be delayed by up to that amount, and that the longest the event loop can sleep in the operating system’s poll call is that amount. Note that this is what gevent does for libev on Windows too.

  • libuv only supports one io watcher per file descriptor, whereas libev and gevent have always supported many watchers using different settings. The libev behaviour is emulated at the Python level.

  • Looping multiple times and expecting events for the same file descriptor to be raised each time without any data being read or written (as works with libev) does not appear to work correctly on Linux when using gevent.select.poll or a monkey-patched selectors.PollSelector.

  • If anything unexpected happens, libuv likes to abort() the entire process instead of reporting an error. For example, closing a file descriptor it is using in a watcher may cause the entire process to be exited.

  • The order in which timers and other callbacks are invoked may be different than in libev. In particular, timers and IO callbacks happen in a different order, and timers may easily be off by up to half of the nominal 1ms resolution. See issue #1057.

  • There is no support for priorities within classes of watchers. libev has some support for priorities and this is exposed in the low-level gevent API, but it was never documented.

  • Low-level fork and child watchers are not available. gevent emulates these in Python on platforms that supply os.fork(). Child watchers use SIGCHLD, just as on libev, so the same caveats apply.

  • Low-level prepare watchers are not available. gevent uses prepare watchers for internal purposes. If necessary, this could be emulated in Python.


In the various micro-benchmarks gevent has, performance among all three loop implementations is roughly the same. There doesn’t seem to be a clear winner or loser.