Re: [PATCH 0/5] futex: Wakeup optimizations

Davidlohr Bueso
2013-11-23T06:38:09Z

Hi Darren,

On Fri, 2013-11-22 at 21:55 -0800, Darren Hart wrote:
> On Fri, 2013-11-22 at 16:56 -0800, Davidlohr Bueso wrote:
> > We have been dealing with a customer database workload on large
> > 12Tb, 240 core 16 socket NUMA system that exhibits high amounts
> > of contention on some of the locks that serialize internal futex
> > data structures. This workload specially suffers in the wakeup
> > paths, where waiting on the corresponding hb->lock can account for
> > up to ~60% of the time. The result of such calls can mostly be
> > classified as (i) nothing to wake up and (ii) wakeup large amount
> > of tasks.
>
> With as many cores as you have, have you done any analysis of how
> effective the hashing algorithm is, and would more buckets relieve someHi
> of the contention.... ah, I see below that you did. Nice work.
>
> > Before these patches are applied, we can see this pathological behavior:
> >
> > 37.12% 826174 xxx [kernel.kallsyms] [k] _raw_spin_lock
> > --- _raw_spin_lock
> > |
> > |--97.14%-- futex_wake
> > | do_futex
> > | sys_futex
> > | system_call_fastpath
> > | |
> > | |--99.70%-- 0x7f383fbdea1f
> > | | yyy
> >
> > 43.71% 762296 xxx [kernel.kallsyms] [k] _raw_spin_lock
> > --- _raw_spin_lock
> > |
> > |--53.74%-- futex_wake
> > | do_futex
> > | sys_futex
> > | system_call_fastpath
> > | |
> > | |--99.40%-- 0x7fe7d44a4c05
> > | | zzz
> > |--45.90%-- futex_wait_setup
> > | futex_wait
> > | do_futex
> > | sys_futex
> > | system_call_fastpath
> > | 0x7fe7ba315789
> > | syscall
> >
>
> Sorry to be dense, can you spell out how 60% falls out of these numbers?

By adding the respective percentages of futex_wake()*_raw_spin_lock
calls.

>
> >
> > With these patches, contention is practically non existent:
> >
> > 0.10% 49 xxx [kernel.kallsyms] [k] _raw_spin_lock
> > --- _raw_spin_lock
> > |
> > |--76.06%-- futex_wait_setup
> > | futex_wait
> > | do_futex
> > | sys_futex
> > | system_call_fastpath
> > | |
> > | |--99.90%-- 0x7f3165e63789
> > | | syscall|
> > ...
> > |--6.27%-- futex_wake
> > | do_futex
> > | sys_futex
> > | system_call_fastpath
> > | |
> > | |--54.56%-- 0x7f317fff2c05
> > ...
> >
> > Patches 1 & 2 are cleanups and micro optimizations.
> >
> > Patch 3 addresses the well known issue of the global hash table.
> > By creating a larger and NUMA aware table, we can reduce the false
> > sharing and collisions, thus reducing the chance of different futexes
> > using hb->lock.
> >
> > Patch 4 reduces contention on the corresponding hb->lock by not trying to
> > acquire it if there are no blocked tasks in the waitqueue.
> > This particularly deals with point (i) above, where we see that it is not
> > uncommon for up to 90% of wakeup calls end up returning 0, indicating that no
> > tasks were woken.
>
> Can you determine how much benefit comes from 3 and how much additional
> benefit comes from 4?

While I don't have specific per-patch data, there are indications that
the workload mostly deals with a handful of futexes. So its pretty safe
to assume that patch 4 is the one with the most benefit for _this_
particular workload.

>
> >
> > Patch 5 resurrects a two year old idea from Peter Zijlstra to delay
> > the waking of the blocked tasks to be done without holding the hb->lock:
> > https://lkml.org/lkml/2011/9/14/118
> >
> > This is useful for locking primitives that can effect multiple wakeups
> > per operation and want to avoid the futex's internal spinlock contention by
> > delaying the wakeups until we've released the hb->lock.
> > This particularly deals with point (ii) above, where we can observe that
> > in occasions the wake calls end up waking 125 to 200 waiters in what we believe
> > are RW locks in the application.
> >
> > This patchset has also been tested on smaller systems for a variety of
> > benchmarks, including java workloads, kernel builds and custom bang-the-hell-out-of
> > hb locks programs. So far, no functional or performance regressions have been seen.
> > Furthermore, no issues were found when running the different tests in the futextest
> > suite: http://git.kernel.org/cgit/linux/kernel/git/dvhart/futextest.git/
>
> Excellent. Would you be able to contribute any of these (C only please)
> to the stress test group?
>

Sure.

Thanks,
Davidlohr