Re: [PATCH RFC 1/3] mutex: Make more scalable by doing less atomicoperations

[Ingo Molnar]

* Waiman Long <Waiman.Long@xxxxxx> wrote:

> > Furthermore, since you are seeing this effect so profoundly, have you 
> > considered using another approach, such as queueing all the poll-waiters in 
> > some fashion?
> >
> > That would optimize your workload additionally: removing the 'stampede' of 
> > trylock attempts when an unlock happens - only a single wait-poller would get 
> > the lock.
>
> The mutex code in the slowpath has already put the waiters into a sleep queue 
> and wait up only one at a time.

Yes - but I'm talking about spin/poll-waiters.

> [...] However, there are 2 additional source of mutex lockers besides those in 
> the sleep queue:
>
> 1. New tasks trying to acquire the mutex and currently in the fast path.
> 2. Mutex spinners (CONFIG_MUTEX_SPIN_ON_OWNER on) who are spinning
> on the owner field and ready to acquire the mutex once the owner
> field change.
> 
> The 2nd and 3rd patches are my attempts to limit the second types of mutex 
> lockers.

Even the 1st patch seems to do that, it limits the impact of spin-loopers, right?

I'm fine with patch #1 [your numbers are proof enough that it helps while the low 
client count effect seems to be in the noise] - the questions that seem open to me 
are:

 - Could the approach in patch #1 be further improved by an additional patch that 
   adds queueing to the _spinners_ in some fashion - like ticket spin locks try to
   do in essence? Not queue the blocked waiters (they are already queued), but the
   active spinners. This would have additional benefits, especially with a high
   CPU count and a high NUMA factor, by removing the stampede effect as owners get 
   switched.

 - Why does patch #2 have an effect? (it shouldn't at first glance) It has a 
   non-trivial cost, it increases the size of 'struct mutex' by 8 bytes, which 
   structure is embedded in numerous kernel data structures. When doing 
   comparisons I'd suggest comparing it not to just vanilla, but to a patch that
   only extends the struct mutex data structure (and changes no code) - this
   allows the isolation of cache layout change effects.

 - Patch #3 is rather ugly - and my hope would be that if spinners are queued in
   some fashion it becomes unnecessary.

Thanks,

	Ingo
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