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From: Tomas Vondra <[email protected]>
To: Tom Lane <[email protected]>
Cc: Peter Eisentraut <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: sequences vs. synchronous replication
Date: Tue, 21 Dec 2021 03:49:33 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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On 12/21/21 02:01, Tom Lane wrote:
> Tomas Vondra <[email protected]> writes:
>> OK, I did a quick test with two very simple benchmarks - simple select
>> from a sequence, and 'pgbench -N' on scale 1. Benchmark was on current
>> master, patched means SEQ_LOG_VALS was set to 1.
> 
> But ... pgbench -N doesn't use sequences at all, does it?
> 
> Probably inserts into a table with a serial column would constitute a
> plausible real-world case.
> 

D'oh! For some reason I thought pgbench has a sequence on the history 
table, but clearly I was mistaken. There's another thinko, because after 
inspecting pg_waldump output I realized "SEQ_LOG_VALS 1" actually logs 
only every 2nd increment. So it should be "SEQ_LOG_VALS 0".

So I repeated the test fixing SEQ_LOG_VALS, and doing the pgbench with a 
table like this:

   create table test (a serial, b int);

and a script doing

   insert into test (b) values (1);

The results look like this:

1) select nextval('s');

      clients          1         4
     ------------------------------
      master       39533    124998
      patched       3748      9114
     ------------------------------
      diff          -91%      -93%


2) insert into test (b) values (1);

      clients          1         4
     ------------------------------
      master        3718      9188
      patched       3698      9209
     ------------------------------
      diff            0%        0%

So the nextval() results are a bit worse, due to not caching 1/2 the 
nextval calls. The -90% is roughly expected, due to generating about 32x 
more WAL (and having to wait for commit).

But results for the more realistic insert workload are about the same as 
before (i.e. no measurable difference). Also kinda expected, because 
those transactions have to wait for WAL anyway.

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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