Re: Some interesting results from tweaking spinlocks

Tom Lane wrote:
> I have been experimenting with altering the SPINS_PER_DELAY number in
> src/backend/storage/lmgr/s_lock.c.  My results suggest that the current
> setting of 100 may be too small.
> 
> The attached graph shows pgbench results on the same 4-way Linux box
> I described in my last message.  (The numbers are not exactly comparable
> to the previous graph, because I recompiled with --enable-cassert off
> for this set of runs.)  All runs use current CVS plus the second LWLock
> patch under discussion.
> 
> Evidently, on this hardware and test case the optimal SPINS_PER_DELAY
> value is somewhere in the low thousands, not 100.  I find this rather
> surprising given that spinlocks are never held for more than a few
> dozen instructions, but the results seem quite stable.
> 
> On the other hand, increasing SPINS_PER_DELAY could hardly fail to be
> a loser on a single-CPU machine.
> 
> Would it be worth making this value a GUC parameter, so that it could
> be tuned conveniently on a per-installation basis?

The difference is small, perhaps 15%.  My feeling is that we may want to
start configuring whether we are on a multi-cpu machine and handle thing
differently.  Are there other SMP issues that could be affected by a
single boolean setting?  Is there a way to detect this on postmaster
startup?

My offhand opinion is that we should keep what we have now and start to
think of a more comprehensive solution for 7.3.

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
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