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

On 2016-04-13 11:27:09 -0400, Robert Haas wrote:
> That page is sort of confusing, because it says that platform has
> those things but then says ***, which is footnoted to mean "linux
> kernel emulation available", but it's not too clear whether that
> applies to all atomics or just 8-byte atomics. The operator
> precedence of / (used as a separator) vs. footnotes is not stated.

/ has a higher precedence than footnotes. Not sure how to make that
easily clear. I'm not exactly a mediawiki expert.

> It's also not clear what "linux kernel emulation available" actually
> means. Should we think of those things being fast, or slow?

Slow. It means that the compiler generates a syscall to perform the
atomic. The syscall disables preemption, then performs the actual math,
re-enables preemption, and returns. That's a lot more expensive than a
spinlock. There's

/*
* 64 bit atomics on arm are implemented using kernel fallbacks and might be
* slow, so disable entirely for now.
* XXX: We might want to change that at some point for AARCH64
*/
#define PG_DISABLE_64_BIT_ATOMICS

for that reason (in the current tree, not patch).

The whole fallback facility exists to make it easier to port software to
arm; but I wouldn't want to rely on it if not necessary.

Greetings,

Andres Freund