Horton

Re: Inlining comparators as a performance optimisation

Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>

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From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
To: Simon Riggs <simon@2ndQuadrant.com>
Cc: Peter Geoghegan <peter@2ndquadrant.com>, Tom Lane <tgl@sss.pgh.pa.us>, PG Hackers <pgsql-hackers@postgresql.org>
Date: 2011-09-21T07:08:42Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Speed up conversion of signed integers to C strings.

  2. Remove some unnecessary tests of pgstat_track_counts.

  3. Remove cvs keywords from all files.

  4. Code cleanup for function prototypes: change two K&R-style prototypes

  5. Use Min() instead of min() in qsort, for consistency and to avoid

  6. pgindent run for 8.2.

  7. Switch over to using our own qsort() all the time, as has been proposed

On 21.09.2011 10:01, Simon Riggs wrote:
> On Wed, Sep 21, 2011 at 7:51 AM, Heikki Linnakangas
> <heikki.linnakangas@enterprisedb.com>  wrote:
>> On 21.09.2011 02:53, Peter Geoghegan wrote:
>>>
>>> C stdlib quick-sort time elapsed: 2.092451 seconds
>>> Inline quick-sort time elapsed: 1.587651 seconds
>>>
>>> Does *that* look attractive to you?
>>
>> Not really, to be honest. That's a 25% speedup in pure qsorting speed. How
>> much of a gain in a real query do you expect to get from that, in the best
>> case? There's so many other sources of overhead that I'm afraid this will be
>> lost in the noise. If you find a query that spends, say, 50% of its time in
>> qsort(), you will only get a 12.5% speedup on that query. And even 50% is
>> really pushing it - I challenge you to find a query that spends any
>> significant amount of time qsorting integers.
>
> How about almost every primary index creation?

Nope. Swamped by everything else.

Also note that as soon as the sort grows big enough to not fit in 
maintenance_work_mem, you switch to the external sort algorithm, which 
doesn't use qsort. Perhaps you could do similar inlining in the heap 
sort & merge passes done in the external sort, but it's unlikely to be 
as big a win there.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com