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

From: Ingo Molnar
Date: Wed Apr 10 2013 - 06:28:40 EST

* 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

- 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

- 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.


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