Re: Spinlocks, yet again: analysis and proposed patches

Tom Lane wrote:
> I guess what this means is that there's no real problem with losing the
> cache line while manipulating the LWLock, which is what the patch was
> intended to prevent. Instead, we're paying for swapping two cache lines
> (the spinlock and the LWLock) across processors instead of just one line.
> But that should at worst be a 2x inflation of the time previously spent
> in LWLockAcquire/Release, which is surely not yet all of the application
> ;-). Why the heck is this so bad? Should we expect that apparently
> minor changes in shared data structures might be costing equivalently
> huge penalties in SMP performance elsewhere?
>
> Unfortunately I don't have root on the Opteron and can't run oprofile.
> But I'd really like to see some oprofile stats from these two cases
> so we can figure out what in the world is going on here. Can anyone
> help?

I will try the patch here and see if it gives the same result. If so I could
try to run with oprofile if you can give me a quick start.

Best Regards,
Michael Paesold