From: | Peter Geoghegan <pg(at)heroku(dot)com> |
---|---|
To: | Ants Aasma <ants(dot)aasma(at)eesti(dot)ee> |
Cc: | Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, "Tsunakawa, Takayuki" <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Is the unfair lwlock behavior intended? |
Date: | 2016-05-24 22:34:31 |
Message-ID: | CAM3SWZQQO0yLSZ=1i-juA6VrrriEwOPGm-X7AMv0LXBFsofcrw@mail.gmail.com |
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On Tue, May 24, 2016 at 1:38 PM, Ants Aasma <ants(dot)aasma(at)eesti(dot)ee> wrote:
>> I've already observed such behavior, see [1]. I think that now there is no
>> consensus on how to fix that. For instance, Andres express opinion that
>> this shouldn't be fixed from LWLock side [2].
>> FYI, I'm planning to pickup work on CSN patch [3] for 10.0. CSN should fix
>> various scalability issues including high ProcArrayLock contention.
>
> Some amount of non-fairness is ok, but degrading to the point of
> complete denial of service is not very graceful. I don't think it's
> realistic to hope that all lwlock contention issues will be fixed any
> time soon. Some fallback mechanism would be extremely nice until then.
Jim Gray's paper on the "Convoy phenomenon" remains relevant, decades later:
http://www.msr-waypoint.com/en-us/um/people/gray/papers/Convoy%20Phenomenon%20RJ%202516.pdf
I could believe that there's a case to be made for per-LWLock fairness
characteristics, which may be roughly what Andres meant.
--
Peter Geoghegan
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