From a4b09af3e93707351f1d4b8efafabfa135c0f70b Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Fri, 31 Jul 2015 20:50:35 +0200
Subject: [PATCH] Micro optimize LWLockAttemptLock() a bit.
LWLockAttemptLock pointlessly read the lock's state in every loop
iteration, even though pg_atomic_compare_exchange_u32() returns the old
value. Instead do that only once before the loop iteration.
Additionally there's no need to have the expected_state variable,
old_state mostly had the same value anyway.
Noticed-By: Heikki Linnakangas
Backpatch: 9.5, no reason to let the branches diverge at this point
---
src/backend/storage/lmgr/lwlock.c | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c
index ae03eb14196..687ed6399cb 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/src/backend/storage/lmgr/lwlock.c
@@ -582,29 +582,33 @@ LWLockInitialize(LWLock *lock, int tranche_id)
static bool
LWLockAttemptLock(LWLock *lock, LWLockMode mode)
{
+ uint32 old_state;
+
AssertArg(mode == LW_EXCLUSIVE || mode == LW_SHARED);
+ /*
+ * Read once outside the loop, later iterations will get the newer value
+ * via compare & exchange.
+ */
+ old_state = pg_atomic_read_u32(&lock->state);
+
/* loop until we've determined whether we could acquire the lock or not */
while (true)
{
- uint32 old_state;
- uint32 expected_state;
uint32 desired_state;
bool lock_free;
- old_state = pg_atomic_read_u32(&lock->state);
- expected_state = old_state;
- desired_state = expected_state;
+ desired_state = old_state;
if (mode == LW_EXCLUSIVE)
{
- lock_free = (expected_state & LW_LOCK_MASK) == 0;
+ lock_free = (old_state & LW_LOCK_MASK) == 0;
if (lock_free)
desired_state += LW_VAL_EXCLUSIVE;
}
else
{
- lock_free = (expected_state & LW_VAL_EXCLUSIVE) == 0;
+ lock_free = (old_state & LW_VAL_EXCLUSIVE) == 0;
if (lock_free)
desired_state += LW_VAL_SHARED;
}
@@ -620,7 +624,7 @@ LWLockAttemptLock(LWLock *lock, LWLockMode mode)
* Retry if the value changed since we last looked at it.
*/
if (pg_atomic_compare_exchange_u32(&lock->state,
- &expected_state, desired_state))
+ &old_state, desired_state))
{
if (lock_free)
{
--
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