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Re: concurrent snapshots

From: Ants Aasma <ants(dot)aasma(at)eesti(dot)ee>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: concurrent snapshots
Date: 2011-09-08 21:45:28
Message-ID: CA+CSw_sS8vN2B-aCR35R7CwDyyej2NPHZAMA6t5606FyaovvWw@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, Sep 8, 2011 at 6:46 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> I'm not convinced it's anywhere near that easy.  For one thing, on at
> least one big server I'm playing with, memory latency on shared memory
> is vastly higher (like >10x!) than on backend-local memory due to NUMA
> effects.

I wonder if both the shared mem and non-local memory issue can be
circumvented by using a slru like mechanism as a side channel to
publish taken snapshots and make concurrent xids available with a
sinval/hasmessages like per proc flag in shared memory to notify of
migrated snapshots.

I'll have to think through the space, locking and performance
considerations. That might take a small while though, I just managed
to contract the flu and can't really think straight.

Sorry to waste your time if this whole approach is completely untenable.
It seemed like a interesting topic to sink my teeth in, but in hindsight
seems a bit too much for a first try.

--
Ants Aasma

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