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Commitc01c25f

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Improve spinlock performance for HP-UX, ia64, non-gcc.
At least on this architecture, it's very important to spin on anon-atomic instruction and only retry the atomic once it appearsthat it will succeed. To fix this, split TAS() into two macros:TAS(), for trying to grab the lock the first time, and TAS_SPIN(),for spinning until we get it. TAS_SPIN() defaults to same as TAS(),but we can override it when we know there's a better way.It's likely that some of the other cases in s_lock.h requiresimilar treatment, but this is the only one we've got conclusiveevidence for at present.
1 parent6e1f1fe commitc01c25f

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2 files changed

+25
-12
lines changed

2 files changed

+25
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lines changed

‎src/backend/storage/lmgr/s_lock.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -96,7 +96,7 @@ s_lock(volatile slock_t *lock, const char *file, int line)
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intdelays=0;
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intcur_delay=0;
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99-
while (TAS(lock))
99+
while (TAS_SPIN(lock))
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{
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/* CPU-specific delay each time through the loop */
102102
SPIN_DELAY();

‎src/include/storage/s_lock.h

Lines changed: 24 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -31,25 +31,33 @@
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*macros at the bottom of the file. Check if your platform can use
3232
*these or needs to override them.
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*
34-
* Usually, S_LOCK() is implemented in terms ofaneven lower-levelmacro
35-
*TAS():
34+
* Usually, S_LOCK() is implemented in terms of even lower-levelmacros
35+
*TAS() and TAS_SPIN():
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*
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*int TAS(slock_t *lock)
3838
*Atomic test-and-set instruction. Attempt to acquire the lock,
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*but do *not* wait.Returns 0 if successful, nonzero if unable
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*to acquire the lock.
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*
42-
*TAS() is NOT part of the API, and should never be called directly.
42+
*int TAS_SPIN(slock_t *lock)
43+
*Like TAS(), but this version is used when waiting for a lock
44+
*previously found to be contended. Typically, this is the
45+
*same as TAS(), but on some architectures it's better to poll a
46+
*contended lock using an unlocked instruction and retry the
47+
*atomic test-and-set only when it appears free.
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*
44-
*CAUTION: on some platforms TAS() may sometimes report failure to acquire
45-
*a lock even when the lock is not locked. For example, on Alpha TAS()
46-
*will "fail" if interrupted. Therefore TAS() should always be invoked
47-
*in a retry loop, even if you are certain the lock is free.
49+
*TAS() and TAS_SPIN() are NOT part of the API, and should never be called
50+
*directly.
4851
*
49-
*ANOTHER CAUTION: be sure that TAS() and S_UNLOCK() represent sequence
50-
*points, ie, loads and stores of other values must not be moved across
51-
*a lock or unlock. In most cases it suffices to make the operation be
52-
*done through a "volatile" pointer.
52+
*CAUTION: on some platforms TAS() and/or TAS_SPIN() may sometimes report
53+
*failure to acquire a lock even when the lock is not locked. For example,
54+
*on Alpha TAS() will "fail" if interrupted. Therefore a retry loop must
55+
*always be used, even if you are certain the lock is free.
56+
*
57+
*ANOTHER CAUTION: be sure that TAS(), TAS_SPIN(), and S_UNLOCK() represent
58+
*sequence points, ie, loads and stores of other values must not be moved
59+
*across a lock or unlock. In most cases it suffices to make the operation
60+
*be done through a "volatile" pointer.
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*
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*On most supported platforms, TAS() uses a tas() function written
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*in assembly language to execute a hardware atomic-test-and-set
@@ -727,6 +735,7 @@ typedef unsigned int slock_t;
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728736
#include<ia64/sys/inline.h>
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#defineTAS(lock) _Asm_xchg(_SZ_W, lock, 1, _LDHINT_NONE)
738+
#defineTAS_SPIN(lock) (*(lock) ? 1 : TAS(lock))
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#endif/* HPUX on IA64, non gcc */
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@@ -925,6 +934,10 @@ extern inttas(volatile slock_t *lock);/* in port/.../tas.s, or
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#defineTAS(lock)tas(lock)
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#endif/* TAS */
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937+
#if !defined(TAS_SPIN)
938+
#defineTAS_SPIN(lock)TAS(lock)
939+
#endif/* TAS_SPIN */
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928941

929942
/*
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* Platform-independent out-of-line support routines

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