|
66 | 66 | * Portions Copyright (c) 1996-2004, PostgreSQL Global Development Group |
67 | 67 | * Portions Copyright (c) 1994, Regents of the University of California |
68 | 68 | * |
69 | | - * $PostgreSQL: pgsql/src/include/storage/s_lock.h,v 1.131 2004/09/24 01:48:43 tgl Exp $ |
| 69 | + * $PostgreSQL: pgsql/src/include/storage/s_lock.h,v 1.132 2004/10/06 23:41:59 neilc Exp $ |
70 | 70 | * |
71 | 71 | *------------------------------------------------------------------------- |
72 | 72 | */ |
@@ -138,6 +138,29 @@ tas(volatile slock_t *lock) |
138 | 138 | static __inline__void |
139 | 139 | spin_delay(void) |
140 | 140 | { |
| 141 | +/* |
| 142 | + * This sequence is equivalent to the PAUSE instruction ("rep" is |
| 143 | + * ignored by old IA32 processors if the following instruction is |
| 144 | + * not a string operation); the IA-32 Architecture Software |
| 145 | + * Developer's Manual, Vol. 3, Section 7.7.2 describes why using |
| 146 | + * PAUSE in the inner loop of a spin lock is necessary for good |
| 147 | + * performance: |
| 148 | + * |
| 149 | + * The PAUSE instruction improves the performance of IA-32 |
| 150 | + * processors supporting Hyper-Threading Technology when |
| 151 | + * executing spin-wait loops and other routines where one |
| 152 | + * thread is accessing a shared lock or semaphore in a tight |
| 153 | + * polling loop. When executing a spin-wait loop, the |
| 154 | + * processor can suffer a severe performance penalty when |
| 155 | + * exiting the loop because it detects a possible memory order |
| 156 | + * violation and flushes the core processor's pipeline. The |
| 157 | + * PAUSE instruction provides a hint to the processor that the |
| 158 | + * code sequence is a spin-wait loop. The processor uses this |
| 159 | + * hint to avoid the memory order violation and prevent the |
| 160 | + * pipeline flush. In addition, the PAUSE instruction |
| 161 | + * de-pipelines the spin-wait loop to prevent it from |
| 162 | + * consuming execution resources excessively. |
| 163 | + */ |
141 | 164 | __asm__ __volatile__( |
142 | 165 | " rep; nop\n"); |
143 | 166 | } |
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