Re: Some interesting results from tweaking spinlocks

[2002-01-05 00:00] Tom Lane said:
| Bruce Momjian <pgman@candle.pha.pa.us> writes:
| > OK, I am a little confused now.  I thought the spinlock was only done a
| > few times if we couldn't get a lock, and if we don't we go to sleep, and
| > the count determines how many times we try.  Isn't that expected to
| > affect SMP machines?
| 
| Yeah, but if the spinlock is only held for a few dozen instructions,
| one would think that the max useful delay is also a few dozen
| instructions (or maybe a few times that, allowing for the possibility
| that other processors might claim the lock before we can get it).
| If we spin for longer than that, the obvious conclusion is that the
| spinlock is held by a process that's lost the CPU, and we should
| ourselves yield the CPU so that it can run again.  Further spinning
| just wastes CPU time that might be used elsewhere.
| 
| These measurements seem to say there's a flaw in that reasoning.
| What is the flaw?

Knowing very little of SMP, it looks like the spinning is parallelizing
as expected, getting to select() faster, then serializing on the 
select() call.  I suspect using usleep() instead of select() might 
relieve the serialization.  I'm aware that usleep(10) will actually 
yield between 10 and 20us due to the kernel's scheduler.
 b

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