Messages in this thread | | | Date | Mon, 22 Jun 2020 21:31:04 +0800 | From | Aaron Lu <> | Subject | Re: [RFC PATCH 0/3 v2] futex: introduce FUTEX_SWAP operation |
| |
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@google.com> > 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
| |