svn commit: r242014 - head/sys/kern

[Andre Oppermann]

On 24.10.2012 21:06, Attilio Rao wrote:
> On Wed, Oct 24, 2012 at 8:00 PM, Jim Harris <jim.harris at gmail.com> wrote:
>> On Wed, Oct 24, 2012 at 11:43 AM, John Baldwin <jhb at freebsd.org> wrote:
>>> On Wednesday, October 24, 2012 2:36:41 pm Jim Harris wrote:
>>>> Author: jimharris
>>>> Date: Wed Oct 24 18:36:41 2012
>>>> New Revision: 242014
>>>> URL: http://svn.freebsd.org/changeset/base/242014
>>>>
>>>> Log:
>>>>    Pad tdq_lock to avoid false sharing with tdq_load and tdq_cpu_idle.
>>>>
>>>>    This enables CPU searches (which read tdq_load) to operate independently
>>>>    of any contention on the spinlock.  Some scheduler-intensive workloads
>>>>    running on an 8C single-socket SNB Xeon show considerable improvement with
>>>>    this change (2-3% perf improvement, 5-6% decrease in CPU util).
>>>>
>>>>    Sponsored by:       Intel
>>>>    Reviewed by:        jeff
>>>>
>>>> Modified:
>>>>    head/sys/kern/sched_ule.c
>>>>
>>>> Modified: head/sys/kern/sched_ule.c
>>>>
>>> ==============================================================================
>>>> --- head/sys/kern/sched_ule.c Wed Oct 24 18:33:44 2012        (r242013)
>>>> +++ head/sys/kern/sched_ule.c Wed Oct 24 18:36:41 2012        (r242014)
>>>> @@ -223,8 +223,13 @@ static int sched_idlespinthresh = -1;
>>>>    * locking in sched_pickcpu();
>>>>    */
>>>>   struct tdq {
>>>> -     /* Ordered to improve efficiency of cpu_search() and switch(). */
>>>> +     /*
>>>> +      * Ordered to improve efficiency of cpu_search() and switch().
>>>> +      * tdq_lock is padded to avoid false sharing with tdq_load and
>>>> +      * tdq_cpu_idle.
>>>> +      */
>>>>        struct mtx      tdq_lock;               /* run queue lock. */
>>>> +     char            pad[64 - sizeof(struct mtx)];
>>>
>>> Can this use 'tdq_lock __aligned(CACHE_LINE_SIZE)' instead?
>>>
>>
>> No - that doesn't pad it.  I believe that only works if it's global,
>> i.e. not part of a data structure.
>
> As I've already said in another thread __align() doesn't work on
> object declaration, so what that won't pad it either if it is global
> or part of a struct.
> It is just implemented as __attribute__((aligned(X))):
> http://gcc.gnu.org/onlinedocs/gcc-3.2/gcc/Type-Attributes.html

Actually it seems gcc itself doesn't really care and it up to the
linker to honor that.

-- 
Andre