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

On Tue, Apr 12, 2016 at 2:28 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2016-04-12 14:17:12 -0500, Kevin Grittner wrote:
>> Well, something is different between your environment and mine,
>> since I saw no difference at scale 100 and 2.2% at scale 200.
>
> In a readonly test or r/w?

Readonly with client and job counts matching scale.

> A lot of this will be different between
> single-socket and multi-socket servers; as soon as you have the latter
> the likelihood of contention being bad goes up dramatically.

Yeah, I know, and 4 socket has been at least an order of magnitude
more problematic in my experience than 2 socket. And the problems
are far, far, far worse on kernels prior to 3.8, especially on 3.x
before 3.8, so it's hard to know how to take any report of problems
on a 4 node NUMA machine without knowing the kernel version.

>> knowing more about your hardware, OS, configuration, etc., might
>> allow me to duplicate a problem so I can fix
>
>> For example, I used a "real" pg config, like I would for a production
>> machine (because that seems to me to be the environment that is most
>> important): the kernel is 3.13 (not one with pessimal scheduling) and
>> has tuning for THP, the deadline scheduler, the vm.*dirty* settings,
>> etc. Without knowing even the kernel and what tuning the OS and pg
>> have had on your box, I could take a lot of shots in the dark without
>> hitting anything.
>
> That shouldn't really matter much for a read-only, shared_buffer
> resident, test? There's no IO and THP pretty much plays no role because
> there's very few memory allocations (removing the pressure causing the
> well known degradations).

I hate to assume which differences matter without trying, but some
of them seem less probable than others.

>> Oh, and the output of `numactl --hardware` would be good to have.
>> Thanks for all information you can provide.
>
> That was on Alexander's/PgPro's machine. Numactl wasn't installed, and I
> didn't have root. But it has four numa domains (gathered via /sys/).

On the machines I've used, it will give you the hardware report
without being root. But of course, it can't do that if it's not
installed. I hadn't yet seen a machine with multiple NUMA memory
segments that didn't have the numactl executable installed; I'll
keep in mind that can happen.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company