@@ -28,4 +28,6 @@ struct mutex_config
extern struct mutex_config __mutex_aconf attribute_hidden;
+#define READ_ONLY_SPIN 1
+
#endif
@@ -124,8 +124,14 @@ __pthread_mutex_lock (pthread_mutex_t *mutex)
if (LLL_MUTEX_TRYLOCK (mutex) != 0)
{
int cnt = 0;
+#ifdef READ_ONLY_SPIN
+ int val = 0;
+ int max_cnt = MIN (__mutex_aconf.spin_count,
+ mutex->__data.__spins * 2 + 10);
+#else
int max_cnt = MIN (MAX_ADAPTIVE_COUNT,
mutex->__data.__spins * 2 + 10);
+#endif
do
{
if (cnt++ >= max_cnt)
@@ -133,7 +139,16 @@ __pthread_mutex_lock (pthread_mutex_t *mutex)
LLL_MUTEX_LOCK (mutex);
break;
}
+#ifdef READ_ONLY_SPIN
+ do
+ {
+ atomic_spin_nop ();
+ val = atomic_load_relaxed (&mutex->__data.__lock);
+ }
+ while (val != 0 && ++cnt < max_cnt);
+#else
atomic_spin_nop ();
+#endif
}
while (LLL_MUTEX_TRYLOCK (mutex) != 0);
@@ -19,4 +19,5 @@
already elided locks. */
#include <elision-conf.h>
+#include <nptl/pthread_mutex_conf.h>
#include <nptl/pthread_mutex_cond_lock.c>
@@ -19,4 +19,5 @@
#include <elision-conf.h>
#include "force-elision.h"
+#include "nptl/pthread_mutex_conf.h"
#include "nptl/pthread_mutex_lock.c"
The pthread adaptive spin mutex spins on the lock for a while before calling into the kernel to block. But, in the current implementation of spinning, the spinners go straight back to LLL_MUTEX_TRYLOCK(cmpxchg) when the lock is contended, it is not a good idea on many targets as that will force expensive memory synchronization among processors and penalize other running threads. For example, it constantly floods the system with "read for ownership" requests, which are much more expensive to process than a single read. Thus, we only use MO read until we observe the lock to not be acquired anymore, as suggeusted by Andi Kleen. Performance impact: Significant mutex performance improvement is not expected for this patch, though, it probably bring some benefit for the scenarios with severe lock contention on many architectures, the whole system performance can benefit from this modification because a number of unnecessary "read for ownership" requests which stress the cache system by broadcasting cache line invalidity are eliminated during spinning. Meanwhile, it may have some tiny performance regression on the lock holder transformation for the case of lock acquisition via spinning gets, because the lock state is checked before acquiring the lock via trylock. Similar mechanism has been implemented for pthread spin lock. Test machine: 2-sockets Skylake platform, 112 cores with 62G RAM Test case: mutex-adaptive-thread (Contended pthread adaptive spin mutex with global update) Usage: make bench BENCHSET=mutex-adaptive-thread Test result: +----------------+-----------------+-----------------+------------+ | Configuration | Base | Head | % Change | | | Total iteration | Total iteration | base->head | +----------------+-----------------+-----------------+------------+ | | Critical section size: 1x | +----------------+------------------------------------------------+ |1 thread | 2.76681e+07 | 2.7965e+07 | +1.1% | |2 threads | 3.29905e+07 | 3.55279e+07 | +7.7% | |3 threads | 4.38102e+07 | 3.98567e+07 | -9.0% | |4 threads | 1.72172e+07 | 2.09498e+07 | +21.7% | |28 threads | 1.03732e+07 | 1.05133e+07 | +1.4% | |56 threads | 1.06308e+07 | 5.06522e+07 | +14.6% | |112 threads | 8.55177e+06 | 1.02954e+07 | +20.4% | +----------------+------------------------------------------------+ | | Critical section size: 10x | +----------------+------------------------------------------------+ |1 thread | 1.57006e+07 | 1.54727e+07 | -1.5% | |2 threads | 1.8044e+07 | 1.75601e+07 | -2.7% | |3 threads | 1.35634e+07 | 1.46384e+07 | +7.9% | |4 threads | 1.21257e+07 | 1.32046e+07 | +8.9% | |28 threads | 8.09593e+06 | 1.02713e+07 | +26.9% | |56 threads | 9.09907e+06 | 4.16203e+07 | +16.4% | |112 threads | 7.09731e+06 | 8.62406e+06 | +21.5% | +----------------+------------------------------------------------+ | | Critical section size: 100x | +----------------+------------------------------------------------+ |1 thread | 2.87116e+06 | 2.89188e+06 | +0.7% | |2 threads | 2.23409e+06 | 2.24216e+06 | +0.4% | |3 threads | 2.29888e+06 | 2.29964e+06 | +0.0% | |4 threads | 2.26898e+06 | 2.21394e+06 | -2.4% | |28 threads | 1.03228e+06 | 1.0051e+06 | -2.6% | |56 threads | 1.02953 +06 | 1.6344e+07 | -2.3% | |112 threads | 1.01615e+06 | 1.00134e+06 | -1.5% | +----------------+------------------------------------------------+ | | Critical section size: 1000x | +----------------+------------------------------------------------+ |1 thread | 316392 | 315635 | -0.2% | |2 threads | 302806 | 303469 | +0.2% | |3 threads | 298506 | 294281 | -1.4% | |4 threads | 292037 | 289945 | -0.7% | |28 threads | 155188 | 155250 | +0.0% | |56 threads | 190657 | 183106 | -4.0% | |112 threads | 210818 | 220342 | +4.5% | +----------------+-----------------+-----------------+------------+ * nptl/pthread_mutex_lock.c: Optimize adaptive spin mutex * nptl/pthread_mutex_conf.h: Add READ_ONLY_SPIN micro definition * sysdeps/unix/sysv/linux/x86/pthread_mutex_lock.c: Enable read only while spinning for x86 architecture * sysdeps/unix/sysv/linux/x86/pthread_mutex_cond_lock.c: Likewise ChangLog: V3->V4: a) Make the optimization opt-in, and enable for x86 architecture as default, as suggested by Florian Weimer. V2->V3: a) Drop the idea of blocking spinners if fail to acquire a lock, since this idea would not be an universal winner. E.g. several threads contend for a lock which protects a small critical section, thus, probably any thread can acquire the lock via spinning. b) Fix the format issue AFAIC V1->V2: fix format issue Suggested-by: Andi Kleen <andi.kleen@intel.com> Signed-off-by: Kemi Wang <kemi.wang@intel.com> --- nptl/pthread_mutex_conf.h | 2 ++ nptl/pthread_mutex_lock.c | 15 +++++++++++++++ sysdeps/unix/sysv/linux/x86/pthread_mutex_cond_lock.c | 1 + sysdeps/unix/sysv/linux/x86/pthread_mutex_lock.c | 1 + 4 files changed, 19 insertions(+)