Re: Replication identifiers, take 4

On 2015-04-07 17:08:16 +0200, Andres Freund wrote:
> I'm starting benchmarks now.

What I'm benchmarking here is the WAL overhead, since that's what we're
debating.

The test setup I used was a pgbench scale 10 instance. I've run with
full_page_write=off to have more reproducible results. This of course
over-emphasizes the overhead a bit. But for a long checkpoint interval
and a memory resident working set it's not that unrealistic.

I ran 50k transactions in a signle b
baseline:
- 20445024
- 20437128
- 20436864
- avg: 20439672
extern 2byte identifiers:
- 23318368
- 23148648
- 23128016
- avg: 23198344
- avg overhead: 13.5%
padding 2byte identifiers:
- 21160408
- 21319720
- 21164280
- avg: 21214802
- avg overhead: 3.8%
extern 4byte identifiers:
- 23514216
- 23540128
- 23523080
- avg: 23525808
- avg overhead: 15.1%

To me that shows pretty clearly that a) reusing the padding is
worthwhile b) even without that using 2byte instead of 4 byte
identifiers is beneficial.

Now. Especially in the case of extern identifiers we *can* optimize a
bit more. But there's no way we can get the efficiency of the version
reusing padding.

To run the benchmarks you need to
SELECT pg_replication_identifier_create('frak');
before starting pgbench with the attached file.

Greetings,

Andres Freund