Re: parallel mode and parallel contexts

From: Andres Freund <andres(at)2ndquadrant(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: parallel mode and parallel contexts
Date: 2015-01-04 00:31:22
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On 2014-12-22 14:14:31 -0500, Robert Haas wrote:
> On Fri, Dec 19, 2014 at 8:59 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > And here is a new version.
> Here is another new version, with lots of bugs fixed.

A couple remarks:
* Shouldn't this provide more infrastructure to deal with the fact that
we might get less parallel workers than we had planned for?
* I wonder if parallel contexts shouldn't be tracked via resowners
* combocid.c should error out in parallel mode, as insurance
* I doubt it's a good idea to allow heap_insert, heap_inplace_update,
index_insert. I'm not convinced that those will be handled correct and
relaxing restrictions is easier than adding them.
* I'd much rather separate HandleParallelMessageInterrupt() in one
variant that just tells the machinery there's a interrupt (called from
signal handlers) and one that actually handles them. We shouldn't even
consider adding any new code that does allocations, errors and such in
signal handlers. That's just a *bad* idea.
* I'm not a fan of the shm_toc stuff... Verbose and hard to read. I'd
much rather place a struct there and be careful about not using
pointers. That also would obliviate the need to reserve some ids.
* Doesn't the restriction in GetSerializableTransactionSnapshotInt()
apply for repeatable read just the same?
* I'm not a fan of relying on GetComboCommandId() to restore things in
the same order as before.
* I'd say go ahead and commit the BgworkerByOid thing
earlier/independently. I've seen need for that a couple times.
* s/entrypints/entrypoints/


Andres Freund

Andres Freund
PostgreSQL Development, 24x7 Support, Training & Services

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