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

On 2016-04-13 10:31:19 -0500, Kevin Grittner wrote:
> With a real-world application with realistic simulated user load
> there was no such regression and a big gain in performance over
> time, so we're talking about adjusting how broad a range of
> workloads it benefits.

I think it depends very heavily on the type of application. To be
affected you need a high rate of snapshot acquisitions. So lots of small
statements, or possibly longer running stuff involving volatile
functions (which IIRC get new snapshots continually).

> but as an example, if I only see such regression on a Linux kernel
> with version a version < 3.8 I am going to be less concerned about
> getting something into 9.6, since IMO it is completely irresponsible
> to run a NUMA machine with 4 or more nodes on an OS with a substandard
> NUMA scheduler.  I'm not sure when 3.8 became available, but according
> to Wikipedia Version 3.10 of the Linux kernel was released in June
> 2013, so it's not like you need to be on the bleeding edge to have a
> decent scheduler.

I don't think effect of adding a single spinlock (an exclusive lock!) in
a hot path is likely to be hugely dependant on the kernel version.
We've had such cases before, and felt the pain. E.g. the spinlock in the
ProcArrayLock used to be a *HUGE* contention point, and it has pretty
much the same acquisition pattern as this spinlock now.

Greetings,

Andres Freund