| From: | Andres Freund <andres(at)2ndquadrant(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Reasoning behind LWLOCK_PADDED_SIZE/increase it to a full cacheline |
| Date: | 2013-09-24 10:48:11 |
| Message-ID: | 20130924104811.GA11964@awork2.anarazel.de |
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| Lists: | pgsql-hackers |
On 2013-09-24 12:39:39 +0200, Tom Lane wrote:
> Andres Freund <andres(at)2ndquadrant(dot)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 & Services
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