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

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Mon, Apr 8, 2013 at 5:42 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > AFAICS the main performance trade-off is the following: when the owner CPU unlocks
> > the mutex, we'll poll it via a read first, which turns the cacheline into
> > shared-read MESI state. Then we notice that its content signals 'lock is
> > available', and we attempt the trylock again.
> >
> > This increases lock latency in the few-contended-tasks case slightly - and we'd
> > like to know by precisely how much, not just for a generic '10-100 users' case
> > which does not tell much about the contention level.
> 
> We had this problem for *some* lock where we used a "read + cmpxchg" in the 
> hotpath and it caused us problems due to two cacheline state transitions (first 
> to shared, then to exclusive). It was faster to just assume it was unlocked and 
> try to do an immediate cmpxchg.
> 
> But iirc it is a non-issue for this case, because this is only about the 
> contended slow path.
> 
> I forget where we saw the case where we should *not* read the initial value, 
> though. Anybody remember?

I had this vague recollection too - and some digging suggests that it might have 
been this discussion on lkml about 3 years ago:

  [RFC][PATCH 6/8] mm: handle_speculative_fault()

These numbers PeterZ ran:

  http://lkml.indiana.edu/hypermail/linux/kernel/1001.1/00170.html

Appear to show such an effect, on a smaller NUMA system.

( But I'm quite sure it came up somewhere else as well, just cannot place it.
  Probabilistic biological search indices are annoying.)

Thanks,

	Ingo
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