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

Darren Hart
2013-11-23T05:55:51Z

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 some
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?

>
> 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?

>
> 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?

>
> This patchset applies on top of Linus' tree as of v3.13-rc1.
>
> Special thanks to Scott Norton, Tom Vanden and Mark Ray for help presenting,
> debugging and analyzing the data.
>
> futex: Misc cleanups
> futex: Check for pi futex_q only once
> futex: Larger hash table
> futex: Avoid taking hb lock if nothing to wakeup
> sched,futex: Provide delayed wakeup list
>
> include/linux/sched.h | 41 ++++++++++++++++++
> kernel/futex.c | 113 +++++++++++++++++++++++++++-----------------------
> kernel/sched/core.c | 19 +++++++++
> 3 files changed, 122 insertions(+), 51 deletions(-)
>

--
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel