Re: RFC: Ideal Adaptive Spinning Conditions

[Avi Kivity <avi@redhat.com>]

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:


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

This simulates a lock hold ratio of 10% with the lock hold time 
exceeding the acquisition time.  Will be interesting to lower both loop 
bounds as well.

-- 
error compiling committee.c: too many arguments to function