This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [PATCH 3/3] Mutex: Avoid useless spinning
- From: kemi <kemi dot wang at intel dot com>
- To: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>, libc-alpha at sourceware dot org
- Cc: Andi Kleen <andi dot kleen at intel dot com>, "Chen, Tim C" <tim dot c dot chen at intel dot com>, Dave <dave dot hansen at linux dot intel dot com>, "ying.huang" <ying dot huang at intel dot com>, aaron <aaron dot lu at intel dot com>, aubrey <aubrey dot li at intel dot com>
- Date: Sun, 8 Apr 2018 16:31:04 +0800
- Subject: Re: [PATCH 3/3] Mutex: Avoid useless spinning
- References: <1522394093-9835-1-git-send-email-kemi.wang@intel.com> <1522394093-9835-3-git-send-email-kemi.wang@intel.com> <44b112b8-829e-bcfd-4eac-fdd8362862ca@linaro.org>
On 2018年04月06日 04:59, Adhemerval Zanella wrote:
>
>
> On 30/03/2018 04:14, Kemi Wang wrote:
>> Usually, we can't set too short time out while spinning on the lock, that
>> probably makes a thread which is trying to acquire the lock go to sleep
>> quickly, thus weakens the benefit of pthread adaptive spin lock.
>>
>> However, there is also a problem if we set the time out large in
>> case of protecting a small critical section with severe lock contention.
>> As we can see the test result in the last patch, the performance is highly
>> effected by the spin count tunables, smaller spin count, better performance
>> improvement. This is because the thread probably spins on the lock until
>> timeout in severe lock contention before going to sleep.
>>
>> In this patch, we avoid the useless spin by making the spinner sleep
>> if it fails to acquire the lock when the lock is available, as suggested
>> by Tim Chen.
>>
>> nr_threads base COUNT=1000(head~1) COUNT=1000(head)
>> 1 51644585 51323778(-0.6%) 51378551(-0.5%)
>> 2 7914789 9867343(+24.7%) 11503559(+45.3%)
>> 7 1687620 3430504(+103.3%) 7817383(+363.2%)
>> 14 1026555 1843458(+79.6%) 7360883(+617.0%)
>> 28 962001 681965(-29.1%) 5681945(+490.6%)
>> 56 883770 364879(-58.7%) 3416068(+286.5%)
>> 112 1150589 415261(-63.9%) 3255700(+183.0%)
>
> As before [2], I checked the change on a 64 cores aarch64 machine, but
> differently than previous patch this one seems to show improvements:
>
> nr_threads base head(SPIN_COUNT=10) head(SPIN_COUNT=1000)
> 1 27566206 28776779 (4.206770) 28778073 (4.211078)
> 2 8498813 9129102 (6.904173) 7042975 (-20.670782)
> 7 5019434 5832195 (13.935765) 5098511 (1.550982)
> 14 4379155 6507212 (32.703053) 5200018 (15.785772)
> 28 4397464 4584480 (4.079329) 4456767 (1.330628)
> 56 4020956 3534899 (-13.750237) 4096197 (1.836850)
>
> I would suggest you to squash both patch in only one for version 2.
>
OK, the separation here is easy for review.
>>
>> Suggested-by: Tim Chen <tim.c.chen@intel.com>
>> Signed-off-by: Kemi Wang <kemi.wang@intel.com>
>> ---
>> nptl/pthread_mutex_lock.c | 25 +++++++++++--------------
>> 1 file changed, 11 insertions(+), 14 deletions(-)
>>
>> diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
>> index c3aca93..0faee1a 100644
>> --- a/nptl/pthread_mutex_lock.c
>> +++ b/nptl/pthread_mutex_lock.c
>> @@ -127,22 +127,19 @@ __pthread_mutex_lock (pthread_mutex_t *mutex)
>> int cnt = 0;
>> int max_cnt = MIN (__mutex_aconf.spin_count,
>> mutex->__data.__spins * 2 + 100);
>> +
>> + /* MO read while spinning */
>> do
>> - {
>> - if (cnt >= max_cnt)
>> - {
>> - LLL_MUTEX_LOCK (mutex);
>> - break;
>> - }
>> - /* MO read while spinning */
>> - do
>> - {
>> - atomic_spin_nop ();
>> - }
>> - while (atomic_load_relaxed (&mutex->__data.__lock) != 0 &&
>> + {
>> + atomic_spin_nop ();
>> + }
>> + while (atomic_load_relaxed (&mutex->__data.__lock) != 0 &&
>> ++cnt < max_cnt);
>> - }
>> - while (LLL_MUTEX_TRYLOCK (mutex) != 0);
>> + /* Try to acquire the lock if lock is available or the spin count
>> + * is run out, go to sleep if fails
>> + */
>> + if (LLL_MUTEX_TRYLOCK (mutex) != 0)
>> + LLL_MUTEX_LOCK (mutex);
>>
>> mutex->__data.__spins += (cnt - mutex->__data.__spins) / 8;
>> }
>>
>
> Please fix the format issue here.
>