Re: pgsql: Support parallel btree index builds.

On Tue, Feb 6, 2018 at 10:46 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Sun, Feb 4, 2018 at 1:11 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> I'll be happier about it when the valgrind buildfarm animals are
>>> happy.
>
>> Me too, but it's not clear what the right fix is.  One thing that
>> would help is if you put in an appearance on the thread where this is
>> being discussed and cast a vote.  (Ditto to Andres.)
>
> If you mean do I like fixing this by adding a valgrind suppression,
> no I do not.  Valgrind suppressions are last-resort band-aids IMO,
> to be applied only when it's clearly understood what behavior we're
> masking and why it's more reasonable to mask it than make it better
> defined.  I, at least, don't have that understanding from looking
> at the thread.  For one thing, Peter has not explained why this issue
> appears now with parallel index build when it did not before; it's
> not like logtape.c isn't old enough to vote.

Yeah, he has actually.  In other cases, the buffer is guaranteed to
have been filled at least once (and thus, from valgrind's point of
view, is initialized) because if that weren't going to happen then we
would have not have switched to a tape-sort in the first place.  You
can't set work_mem smaller than 8kB.  But in the parallel case each
worker must always produce a tape, so it can happen if a worker is
unlucky enough to get only a very small slice of the data (because the
other participants gobble it all up before that process really gets
going).

> Even granting that a suppression is the way to fix it, the proposed
> suppression seems pretty darn broad, and hence likely to mask things
> we'd wish it hadn't.

Well, he talked about that at some length too.  I don't know how
you're not seeing it on the thread.  But what I really need here is
some input on an option you do like, not just a list of things you
don't like.

-- 
Robert Haas
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