Re: futex results with dbt-3

Forgive my naivete, but do futex's implement some priority algorithm for
which process gets control. One of the problems as I understand it is
that linux does (did ) not implement a priority algorithm, so it is
possible for the context which just gave up control to be the next
context woken up, which of course is a complete waste of time.

--dc--

Tom Lane wrote:

>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
>
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Dave Cramer
www.postgresintl.com
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