Re: Spinlocks and semaphores in 9.2 and 9.3

Andres Freund <andres@anarazel.de> writes:
> On 2016-04-18 11:24:07 -0400, Tom Lane wrote:
>> (The thing that gave me pause about this was noticing that I could not
>> start two such postmasters concurrently on my RHEL6 box, without changing
>> the default system limits on number of SysV semaphores.)

> Hm, is that different before/after the patch? Because afaics at around
> NBuffers = 1000, the number of semaphores should be lower after the
> patch?

Yeah, that was probably the argument we used before.  But it's true that a
postmaster built with --disable-spinlocks is harder to test than one built
without (because you're going from ~100 semas to ~1100 semas, at default
configuration).  If we believe that the main real use-case for this switch
is testing, that's not a very nice property.

The bottom line IMO is that --disable-spinlocks is actually not that awful
for low-stress, low-concurrency scenarios; for example, it doesn't change
the runtime of make installcheck-parallel very much for me.  On the other
hand, for high-concurrency scenarios you'd darn well better get a real
spinlock implementation.  Given that sort of scope of use, it seems like
a hundred or so underlying semas should be plenty, and it'd be less likely
to cause operational problems than 1024.
        regards, tom lane