From: | Andres Freund <andres(at)anarazel(dot)de> |
---|---|
To: | Kevin Grittner <kgrittn(at)gmail(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Kevin Grittner <kgrittn(at)postgresql(dot)org>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Re: [COMMITTERS] pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold < |
Date: | 2016-04-13 15:59:42 |
Message-ID: | 20160413155942.fx6bnpwpoanbn3i4@alap3.anarazel.de |
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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
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