Re: Re: [COMMITTERS] pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Wed, Apr 13, 2016 at 10:20 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> That's what I thought you were going to say, and it means that any
>> "performance improvement" patch that relies on 64-bit atomics in hotspot
>> code paths is going to be a complete disaster on anything but modern Intel
>> hardware. I'm not sure that's a direction we want to go in. We need to
>> stick to a set of atomics that's pretty widely portable.

> I think 64-bit atomics *are* pretty widely portable. Can you name a
> system with more than 4 CPU cores that doesn't support them?

No, you're ignoring my point, which is what happens on single-CPU
32-bit machines, and whether we aren't going to destroy performance
on low-end machines in pursuit of better performance on high-end.

Now, to the extent that a patch uses a 64-bit atomic op to replace
a spinlock acquisition, it might be pretty much a wash if low-end
machines have to use a spinlock to emulate the atomic op. But it
would be really easy for the translation to replace one spinlock
acquisition with multiple spinlock acquisitions, and that would hurt.

regards, tom lane