Date:	Fri, 22 Nov 2013 21:55:45 -0800
From:	Darren Hart <dvhart@...ux.intel.com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	peterz@...radead.org, tglx@...utronix.de, efault@....de,
	jeffm@...e.com, torvalds@...ux-foundation.org, scott.norton@...com,
	tom.vaden@...com, aswin@...com, Waiman.Long@...com,
	jason.low2@...com
Subject: Re: [PATCH 0/5] futex: Wakeup optimizations

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


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