Re: Switch pfil(9) to rmlocks

On Friday 23 November 2007, Robert Watson wrote:
> On Fri, 23 Nov 2007, Max Laier wrote:
> > attached is a diff to switch the pfil(9) subsystem to rmlocks, which
> > are more suited for the task.  I'd like some exposure before doing
> > the switch, but I don't expect any fallout.  This email is going
> > through the patched pfil already - twice.
>
> Max,
>
> Have you done performance measurements that show rmlocks to be a win in
> this scenario?  I did some patchs for UNIX domain sockets to replace
> the rwlock there but it appeared not to have a measurable impact on SQL
> benchmarks, presumbaly because the read/write blend wasn't right and/or
> that wasnt a significant source of overhead in the benchmark.  I'd
> anticipate a much more measurable improvement for pfil, but would be
> interested in learning how much is seen?

I had to roll an artificial benchmark in order to see a significant change 
(attached - it's a hack!).

Using 3 threads on a 4 CPU machine I get the following results:
null hook: ~13% +/- 2
mtx hook: up to 40% [*]
rw hook: ~5% +/- 1
rm hook: ~35% +/- 5

[*] The mtx hook is inconclusive as my measurements vary a lot.  If one 
thread gets lucky and keeps running the overall time obviously goes down 
by a magnitude.  It seems however, that rmlocks greatly increase the 
chance of that happening - not sure if that's a good thing, though.  If 
all threads receive approximately equal runtime (which is almost always 
the case for rwlocks) the difference is somewhere around 10%.

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