[sync] Optimizing Boost's sync using futex2

From: André Almeida (andrealmeid_at_[hidden])
Date: 2021-05-13 14:27:40

Hi there,

I'm the author of futex2[0], a WIP new set of Linux's syscalls that
allows userspace to write efficient sync mechanisms. I would like to
hear from Boost's developers if the project would benefit from this new
interface.

>From Boost/sync's codebase, I can see that you are already familiar with
futexes, but just in case:

* What's futex?

futex stands for "fast userspace mutexes". It's a syscall that basically
exposes a wait queue, where userspace can ask to put threads to sleep
and to wake them. The index of such queue is an unsigned int address
(that is usually called a futex). It's value doesn't mean anything for
the kernel, userspace is responsible to defining the rules of it and
implementing the logic.

* Why futex2?

futex has long standing issues that seems impossible to be solved in the
current interface, so we are designing a new set of syscalls taking
those problems in account (more on that at [0]). It's not merged yet,
and I'm researching some uses cases to validate and get feedback about
the API. The new API has the following new features:

* Variable sized futexes (6, 16, 32 and 64 bits): we aren't restrict to
only 32 bits anymore, and this can be used for decreasing memory usage
or to wait on an address

* Wait for multiple futexes: this function enables userspace to wait on
a list of futexes and wake on the first that triggers a wake. Our
current use case for this is for game engines and to emulate
WaitForMultipleObjects from WinAPI in Wine

* NUMA-awarenes: current futex has a single global hash table that is
allocated in some node. Every futex() call that happens on the other
nodes will have a access penalty. At futex2, there'll be a single table
per node, so you can specify which one you want to operate on to
increase locality.

The detailed description of the API can be seen in the documentation
patch[1]. Do you think that Boost would benefit from it?

Thanks,
André

- futex2 talk at OpenSource Summit:
https://www.youtube.com/watch?v=eG6GMBTcPQ8

[0]
https://lore.kernel.org/lkml/[e-mail]/

[1]
https://lore.kernel.org/lkml/[e-mail]/