Re: [RFC PATCH 0/3 v2] futex: introduce FUTEX_SWAP operation

[Aaron Lu]

On Tue, Jun 16, 2020 at 10:22:11AM -0700, Peter Oskolkov wrote:
> From 7b091e46de4f9227b5a943e6d78283564e8c1c72 Mon Sep 17 00:00:00 2001
> From: Peter Oskolkov <posk@xxxxxxxxxx>
> Date: Tue, 16 Jun 2020 10:13:58 -0700
> Subject: [RFC PATCH 0/3 v2] futex: introduce FUTEX_SWAP operation
> 
> This is an RFC!
> 
> As Paul Turner presented at LPC in 2013 ...
> - pdf: http://pdxplumbers.osuosl.org/2013/ocw//system/presentations/1653/original/LPC%20-%20User%20Threading.pdf
> - video: https://www.youtube.com/watch?v=KXuZi9aeGTw
> 
> ... Google has developed an M:N userspace threading subsystem backed
> by Google-private SwitchTo Linux Kernel API (page 17 in the pdf referenced
> above). This subsystem provides latency-sensitive services at Google with
> fine-grained user-space control/scheduling over what is running when,
> and this subsystem is used widely internally (called schedulers or fibers).
> 
> This RFC 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.

schbench used futex wait/wake to do sleep/wakeup between message thread
and worker thread and when worker thread is 1 per message thread, the
message thread and worker thread is also flipcall style.

So I modified schbench to make use of futex_swap and did a comparison.
In the not overloaded case, both runs roughly the same with futex_swap
performing slightly better. In the overloaded case, futex_swap performs
better than futex wait/wake in all metrics, with 90th seeing the largest
difference: 2556us vs 6us.

I guess when the scheduler change is in place, more latency gain is
expected.

Here is the log of the schbench run(on a 16core/32cpu x86_64 machine):

overloaded case

original schbench(aka futex wait/wake)
$./schbench -m 64 -t 1 -r 30

Latency percentiles (usec)
        50.0000th: 7
        75.0000th: 9
        90.0000th: 2556
        95.0000th: 7112
        *99.0000th: 14160
        99.5000th: 17504
        99.9000th: 22688
        min=0, max=30351

with futex swap
$./schbench -m 64 -t 1 -r 30

Latency percentiles (usec)
        50.0th: 4
        75.0th: 5
        90.0th: 6
        95.0th: 4568
        *99.0th: 12912
        99.5th: 15152
        99.9th: 20384
        min=0, max=30723


not overloaded case

original schbench(aka futex wait/wake)

$./schbench -m 32 -t 1 -r 30
Latency percentiles (usec)
        50.0000th: 6
        75.0000th: 7
        90.0000th: 8
        95.0000th: 9
        *99.0000th: 10
        99.5000th: 12
        99.9000th: 18
        min=0, max=398


with futex swap

$./schbench -m 32 -t 1 -r 30
Latency percentiles (usec)
        50.0th: 4
        75.0th: 5
        90.0th: 5
        95.0th: 6
        *99.0th: 8
        99.5th: 9
        99.9th: 12
        min=0, max=245