Re: WIP: Fast GiST index build

On 01.09.2011 12:23, Alexander Korotkov wrote:
> On Thu, Sep 1, 2011 at 12:59 PM, Heikki Linnakangas<
> heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>
>> So I changed the test script to generate the table as:
>>
>> CREATE TABLE points AS SELECT random() as x, random() as y FROM
>> generate_series(1, $NROWS);
>>
>> The unordered results are in:
>>
>> testname | nrows | duration | accesses
>> -----------------------------+**-----------+-----------------+**----------
>> points unordered buffered | 250000000 | 05:56:58.575789 | 2241050
>> points unordered auto | 250000000 | 05:34:12.187479 | 2246420
>> points unordered unbuffered | 250000000 | 04:38:48.663952 | 2244228
>>
>> Although the buffered build doesn't lose as badly as it did with more
>> overlap, it still doesn't look good :-(. Any ideas?
>
> But it's still a lot of overlap. It's about 220 accesses per small area
> request. It's about 10 - 20 times greater than should be without overlaps.
> If we roughly assume that 10 times more overlap makes 1/10 of tree to be
> used for actual inserts, then that part of tree can easily fit to the cache.
> You can try my splitting algorithm on your test setup (it this case I advice
> to start from smaller number of rows, 100 M for example).
> I'm requesting real-life datasets which makes troubles in real life from
> Oleg. Probably those datasets is even larger or new linear split produce
> less overlaps on them.

I made a small tweak to the patch, and got much better results (this is
with my original method of generating the data):

testname | nrows | duration | accesses
-----------------------------+-----------+-----------------+----------
points unordered buffered | 250000000 | 03:34:23.488275 | 3945486
points unordered auto | 250000000 | 02:55:10.248722 | 3767548
points unordered unbuffered | 250000000 | 04:02:04.168138 | 4564986

The tweak I made was to the way buffers are emptied in the final
emptying phase. Previously, it repeatedly looped through all the buffers
at a level, until there were no more non-empty buffers at the level.
When a buffer was split while it was being emptied, processing that
buffer stopped, and the emptying process moved on to the next buffer. I
changed it so that when a buffer splits, we continue emptying that
buffer until it's completely empty. That behavior is much more
cache-friendly, which shows as much better overall performance.

I probably changed that behavior for the worse in previous my rounds of
cleanup. Anyway, attached is the patch I used to get the above numbers.
Now that the performance problem is fixed, I'll continue reviewing and
cleaning up the patch.

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