Google Finally Begins Their Open-Source Dance Around Linux User-Space Threading

Way back in 2013 there was a presentation at the Linux Plumbers Conference around Google's work on user-level threads and how they were working on new kernel functionality for using regular threads in a cooperative fashion and building various features off that. Fast forward to today, that functionality has been in use internally at Google for a range of services for latency-sensitive services and greater control over user-space scheduling while now finally in 2020 they are working towards open-sourcing that work.

Following an early request for comments at the end of June, Google has sent out a request for the new "FUTEX_SWAP" functionality to be pulled into Linux 5.9. Google's Peter Oskolkov explained:

This patchset is the first step to open-source this work. As explained in the linked pdf and video, SwitchTo API has three core operations: wait, resume, and swap (=switch). So this patchset adds a FUTEX_SWAP operation that, in addition to FUTEX_WAIT and FUTEX_WAKE, will provide a foundation on top of which user-space threading libraries can be built.

Another common use case for FUTEX_SWAP is message passing a-la RPC between tasks: task/thread T1 prepares a message, wakes T2 to work on it, and waits for the results; when T2 is done, it wakes T1 and waits for more work to arrive. Currently the simplest way to implement this is

a. T1: futex-wake T2, futex-wait
b. T2: wakes, does what it has been woken to do
c. T2: futex-wake T1, futex-wait

With FUTEX_SWAP, steps a and c above can be reduced to one futex operation that runs 5-10 times faster.

A 5~10x speed improvement with FUTEX_SWAP certainly sounds compelling as does the information shared way back at LPC 2013 via the video below and the PDF slides.

While FUTEX_SWAP could be honored for the Linux 5.9 cycle, this is just the start and will likely be a few more cycles before all of this Google work is finally open-source and mainlined. More details via this patch series.