Re: RFC: Ideal Adaptive Spinning Conditions

From: Darren Hart
Date: Thu Apr 01 2010 - 11:55:10 EST

Avi Kivity wrote:
On 04/01/2010 02:21 AM, Darren Hart wrote:
I'm looking at some adaptive spinning with futexes as a way to help reduce the dependence on sched_yield() to implement userspace spinlocks. Chris, I included you in the CC after reading your comments regarding sched_yield() at kernel summit and I thought you might be interested.

I have an experimental patchset that implements FUTEX_LOCK and FUTEX_LOCK_ADAPTIVE in the kernel and use something akin to mutex_spin_on_owner() for the first waiter to spin. What I'm finding is that adaptive spinning actually hurts my particular test case, so I was hoping to poll people for context regarding the existing adaptive spinning implementations in the kernel as to where we see benefit. Under which conditions does adaptive spinning help?

I presume locks with a short average hold time stand to gain the most as the longer the lock is held the more likely the spinner will expire its timeslice or that the scheduling gain becomes noise in the acquisition time. My test case simple calls "lock();unlock()" for a fixed number of iterations and reports the iterations per second at the end of the run. It can run with an arbitrary number of threads as well. I typically run with 256 threads for 10M iterations.

futex_lock: Result: 635 Kiter/s
futex_lock_adaptive: Result: 542 Kiter/s

A lock(); unlock(); loop spends most of its time with the lock held or contended. Can you something like this:

for (i = 0; i < 1000; ++i)
asm volatile ("" : : : "memory");
for (i = 0; i < 10000; ++i)
asm volatile ("" : : : "memory");

Great idea. I'll be doing a more rigorous investigation on this of course, but I thought I'd share the results of just dumping this into the testcase:

# ./futex_lock -i10000000
futex_lock: Measure FUTEX_LOCK operations per second
Arguments: iterations=10000000 threads=256 adaptive=0
Result: 420 Kiter/s
lock calls: 9999872
lock syscalls: 665824 (6.66%)
unlock calls: 9999872
unlock syscalls: 861240 (8.61%)

# ./futex_lock -a -i10000000
futex_lock: Measure FUTEX_LOCK operations per second
Arguments: iterations=10000000 threads=256 adaptive=1
Result: 426 Kiter/s
lock calls: 9999872
lock syscalls: 558787 (5.59%)
unlock calls: 9999872
unlock syscalls: 603412 (6.03%)

This is the first time I've seen adaptive locking have an advantage! The second set of runs showed a slightly greater advantage. Note that this was still with spinners being limited to one.

My thanks to everyone for their insight. I'll be preparing some result matrices and will share the patches and testcases here shortly.

Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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