Re: Reasoning behind LWLOCK_PADDED_SIZE/increase it to a full cacheline

On 2013-09-24 12:39:39 +0200, Tom Lane wrote:
> Andres Freund <andres@2ndquadrant.com> writes:
> > So, what we do is we guarantee that LWLocks are aligned to 16 or 32byte
> > boundaries. That means that on x86-64 (64byte cachelines, 24bytes
> > unpadded lwlock) two lwlocks share a cacheline.

> > In my benchmarks changing the padding to 64byte increases performance in
> > workloads with contended lwlocks considerably.
> 
> At a huge cost in RAM.  Remember we make two LWLocks per shared buffer.

> I think that rather than using a blunt instrument like that, we ought to
> see if we can identify pairs of hot LWLocks and make sure they're not
> adjacent.

That's a good point. What about making all but the shared buffer lwlocks
64bytes? It seems hard to analyze the interactions between all the locks
and keep it maintained.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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