This patch adds a comment that describes what the "rep; no" sequence in
spin_delay() does, in src/include/storage/s_lock.h. I think fully
describing what this code sequence is intended to due is worth the
space, but speak up if you think I've added too large a comment...

Barring any objections, I intend to apply this tomorrow.

-Neil

Index: src/include/storage/s_lock.h
===================================================================
RCS file: /home/neilc/private-cvsroot/pgsql-server/src/include/storage/s_lock.h,v
retrieving revision 1.131
diff -c -r1.131 s_lock.h
*** src/include/storage/s_lock.h	24 Sep 2004 01:48:43 -0000	1.131
--- src/include/storage/s_lock.h	6 Oct 2004 06:48:42 -0000
***************
*** 138,143 ****
--- 138,166 ----
  static __inline__ void
  spin_delay(void)
  {
+ 	/*
+ 	 * This sequence is equivalent to the PAUSE instruction ("rep" is
+ 	 * ignored by old IA32 processors if the following instruction is
+ 	 * not a string operation); the IA-32 Architecture Software
+ 	 * Developer's Manual, Vol. 3, Section 7.7.2 describes why using
+ 	 * PAUSE in the inner loop of a spin lock is necessary for good
+ 	 * performance:
+ 	 *
+ 	 *     The PAUSE instruction improves the performance of IA-32
+ 	 *     processors supporting Hyper-Threading Technology when
+ 	 *     executing spin-wait loops and other routines where one
+ 	 *     thread is accessing a shared lock or semaphore in a tight
+ 	 *     polling loop. When executing a spin-wait loop, the
+ 	 *     processor can suffer a severe performance penalty when
+ 	 *     exiting the loop because it detects a possible memory order
+ 	 *     violation and flushes the core processor's pipeline. The
+ 	 *     PAUSE instruction provides a hint to the processor that the
+ 	 *     code sequence is a spin-wait loop. The processor uses this
+ 	 *     hint to avoid the memory order violation and prevent the
+ 	 *     pipeline flush. In addition, the PAUSE instruction
+ 	 *     de-pipelines the spin-wait loop to prevent it from
+ 	 *     consuming execution resources excessively.
+ 	 */
  	__asm__ __volatile__(
  		" rep; nop			\n");
  }
---------------------------(end of broadcast)---------------------------
TIP 5: Have you checked our extensive FAQ?

               http://www.postgresql.org/docs/faqs/FAQ.html

Reply via email to