Re: Should we cacheline align PGXACT?

On Wed, Feb 15, 2017 at 12:48 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> ... Maybe that difference matters to the memory prefetching
>> controller, I dunno, but it seems funny that we did the PGXACT work to
>> reduce the number of cache lines that had to be touched in order to
>> take a snapshot to improve performance, and now we're talking about
>> increasing it again, also to improve performance.
>
> Yes. I was skeptical that the original change was adequately proven
> to be a good idea, and I'm even more skeptical this time. I think
> every single number that's been reported about this is completely
> machine-specific, and likely workload-specific too, and should not
> be taken as a reason to do anything.

The original change definitely worked on read-only pgbench workloads
on both x86 and Itanium, and the gains were pretty significant at
higher client counts. I don't know whether we tested POWER.
Read-only pgbench throughput is not the world, of course, but it's a
reasonable proxy for high-concurrency, read-mostly workloads involving
short transactions, so it's not nothing, either.

--
Robert Haas
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