* Waiman Long <waiman.l...@hp.com> 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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