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Re: Inlining comparators as a performance optimisation

From: Peter Geoghegan <peter(at)2ndquadrant(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PG Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Inlining comparators as a performance optimisation
Date: 2011-11-23 01:09:23
Message-ID: CAEYLb_WH33La4ebVNqgicmba-2CuexDTiS8cj7M5p3NuLRwgew@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On 21 November 2011 22:55, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Results on my machine, for what they're worth:

I was curious as to how much of an improvement I'd made to sorting in
isolation. I've adding simple sort timing instrumentation to the code
in a revised patch, attached, for the convenience of reviewers.

This patch adds an int8 specialisation, but it also supports fast path
sorting of timestamps by using that same int8 specialisation (for
HAVE_INT64_TIMESTAMP builds at least - didn't know if it was worth
handling the NaN crud for the other case). ISTM that various different
types with identical internal representations can share
specialisations like this, which I hope will alleviate some earlier
concerns about both the limited applicability of this optimisation and
the possible proliferation of specialisations.

Here's some raw qsort figures in seconds for the query "select * from
orderlines order by test" (where test is a timestamptz column I added
and updated with some data, executing repeatedly on the same machine
as before):

Before:

DEBUG:  elapsed: 0.031738
DEBUG:  elapsed: 0.031595
DEBUG:  elapsed: 0.031502
DEBUG:  elapsed: 0.031378
DEBUG:  elapsed: 0.031359
DEBUG:  elapsed: 0.031413
DEBUG:  elapsed: 0.031499
DEBUG:  elapsed: 0.031394
DEBUG:  elapsed: 0.031368

After:

DEBUG:  elapsed: 0.013341
DEBUG:  elapsed: 0.014608
DEBUG:  elapsed: 0.013809
DEBUG:  elapsed: 0.013244
DEBUG:  elapsed: 0.014307
DEBUG:  elapsed: 0.013207
DEBUG:  elapsed: 0.013227
DEBUG:  elapsed: 0.013264
DEBUG:  elapsed: 0.013143
DEBUG:  elapsed: 0.013455
DEBUG:  elapsed: 0.013447

I wonder, is it worth qualifying that the "Sort Method" was a
"quicksort (fast path)" sort within explain analyze output? This is a
rather large improvement, so It might actually be something that
people look out for, as it might be tricky to remember the exact
circumstances under which the optimisation kicks in by the time we're
done here.

I haven't had as much time as I'd like to polish this patch, or to get
clearer answers. I expect to spend more time on it over the weekend.

-- 
Peter Geoghegan       http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services

Attachment: 0001-Initial-commit-of-optimization.patch
Description: text/x-patch (12.3 KB)

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