Manfred Spraul <manfred@colorfullife.com> writes:
> Tom Lane wrote:
>> The bigger problem here is that the SMP locking bottlenecks we are
>> currently seeing are *hardware* issues (AFAICT anyway). The only way
>> that futexes can offer a performance win is if they have a smarter way
>> of executing the basic atomic-test-and-set sequence than we do;
>>
> lwlocks operations are not a basic atomic-test-and-set sequence. They
> are spinlock, several nonatomic operations, spin_unlock.
Right, and it is the spinlock that is the problem. See discussions a
few months back: at least on Intel SMP machines, most of the problem
seems to have to do with trading the spinlock's cache line back and
forth between CPUs. It's difficult to see how a futex is going to avoid
that.
regards, tom lane
