RE: [EXTERNAL] Re: track_io_timing default setting

>-----Original Message-----
>Sent: Friday, December 10, 2021 9:20 AM
>To: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
>Cc: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
>Subject: [EXTERNAL] Re: track_io_timing default setting

>Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:
> Can we change the default setting of track_io_timing to on?

>That adds a very significant amount of overhead on some platforms (gettimeofday is not cheap if it requires a kernel call). And I doubt the claim that the average Postgres user needs this, and doubt even more >that they need it on all the time.
>So I'm -1 on the idea.

regards, tom lane
>

In all honesty, the term "significant amount of overhead on some platforms" is ambiguous. Exactly how much overhead and on what platforms??? I would prefer the document to say something on the order of:

"Enables timing of database I/O calls. This parameter is historically off by default, because it will repeatedly query the operating system for the current time, which may increase overhead costs of elapsed time for each IO. Platforms known to incur a problematic overhead are, <etc, etc, etc>. To measure the overhead of timing on your system, use the pg_test_timing tool. This overhead may become a performance issue when less than 90% of the tests execute for more than 1 microsecond (us). Please refer to the pg_test_timing tool page for more details"

I have the timing always turned on, but that doesn't necessarily mean the default should be changed. However the documentation should be changed as the current phrasing would probably discourage some folks from even trying. I ran the pg_test_timing tool and it came out to .000000023 seconds overhead. Since we typically measure IO in terms of milliseconds, this number is statistically insignificant.

As long as we're on the topic, the documentation for the pg_test_timing tool as well as the output of the tool have something to be desired. The tool output looks like this:

Testing timing overhead for 3 seconds.
Per loop time including overhead: 23.02 ns
Histogram of timing durations:
< us % of total count
1 97.70191 127332403
2 2.29729 2993997
4 0.00007 90
8 0.00069 904
16 0.00004 57

Take note of the comment: "Per loop time including overhead" - so does that means the overhead IS LESS than the reported 23.02 ns? Is that an issue with the actual test code or the output prose? Furthermore the tool's doc goes on to things like this:

"The i7-860 system measured runs the count query in 9.8 ms while the EXPLAIN ANALYZE version takes 16.6 ms, each processing just over 100,000 rows. That 6.8 ms difference means the timing overhead per row is 68 ns, about twice what pg_test_timing estimated it would be. Even that relatively small amount of overhead is making the fully timed count statement take almost 70% longer. On more substantial queries, the timing overhead would be less problematic."

IMHO this is misleading. This timing process is what EXPLAIN ANALYZE does and most likely completely unrelated to the topic in question - that is turning on io timing! What this paragraph is implying through the reader's chain of events is that IF you turn on track_io_timing you may result in a 70% overhead!!! Umm - really???

Long story short, I'm perfectly fine with this 'overhead' - unless someone wants to refute this.
Regards,
phil