Re: spinlock contention

On Jul 12, 2011, at 8:10 PM, Florian Pflug <fgp(at)phlo(dot)org> wrote:
> On Jul13, 2011, at 00:10 , Robert Haas wrote:
>> On Jul 12, 2011, at 8:03 AM, Florian Pflug <fgp(at)phlo(dot)org> wrote:
>>> The algorithm is quite straight forward, if one assumes a lock-free
>>> implementation of a queue (More on that below)
>>
>> This is similar to the CAS-based LWLocks I played around with, though
>> I didn't use a lock-free queue. I think I probably need to devote some
>> time to figuring out why that didn't work out to a win...
>
> Yeah, the non-waitqueue-related parts are mostly identical. The only
> significant difference there is that the waiters-present bit is replaced
> by fence-and-recheck.
>
> What motivated me to research this was your theory that the problem is
> not so much spin-lock contention, but rather those unlucky processes who
> lose their time-slice while holding the lock.
>
> If that is true, then maybe the problem with your CAS patch is that
> once the LWLocks is contested heavily, the waiters-present bit will be
> set pretty much all the time, and the code will thus fall back to
> using the spin-lock.
>
> *Reading the code again*
>
> Hm, I just realized that you only clear the waiters-present bit after
> emptying the queue completely. With rising contention, you might reach a
> point where you never empty the queue completely (unless the lock is
> only ever acquired in share mode, that is). As it stands, the CAS patch
> is effectively neutralized at this point. It'll even have an adverse
> effect due to the higher number of atomic operations compared to the
> unpatched version.

Hmm, yeah.

> I wonder if clearing the waiters-present bit only upon clearing the
> queue completely is necessary for correctness. Wouldn't it be OK
> to clear the bit after waking up at least one locker? That'd still
> guarantee that you cannot end up with a blocked process but no lock
> holder, I believe.

I don't think so - how does the next process that releases the lock know to release waiters?

...Robert