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Micro optimize LWLockAttemptLock() a bit.

author Andres Freund <andres@anarazel.de>

2015年7月31日 18:50:35 +0000 (20:50 +0200)

committer Andres Freund <andres@anarazel.de>

Sun, 2 Aug 2015 16:41:23 +0000 (18:41 +0200)

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 patch | blob | blame | history

diff --git a/src/backend/storage/lmgr/lwlock.c b/src/backend/storage/lmgr/lwlock.c

index ae03eb14196c80fec7a0d042da94726a4a2e2b65..687ed6399cb19ed3183bb75a0214b57229e05675 100644 (file)

--- 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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