Re: review: pgbench progress report improvements

Hello Pavel,

Thanks for your review.

> * patched with minor warning
> * compilable cleanly
> * zero impact on PostgreSQL server functionality
> * it does what was in proposal
> ** change 5sec progress as default (instead no progress)
> ** finalise a rate limit support - fixes a latency calculation

Just a point about the motivation: the rationale for having a continuous
progress report is that benchmarking is subject to possibly long warmup
times, and thus a test may have to run for hours so as to be significant.
I find running a command for hours without any hint about what is going on
quite annoying.

> * code is clean
> * documentation is included
> * there is no voices against this patch and this patch increases a pgbench
> usability/
>
> I have only one question. When I tested this patch with throttling I got a
> very similar values of lag.

Yep. That is just good!

> What is sense, or what is semantic of this value?

The "lag" measures the stochastic processus health. Actually, it measures
how far behind schedule the clients are when performing throttled
transactions. If it was to increase, that would mean that something is
amiss, possibly not enough client threads or other issues. If it is small,
then all is well.

> It is not detailed documented.

It is documented in the section about the --rate option, see
http://www.postgresql.org/docs/devel/static/pgbench.html

> Should be printed this value in this form on every row? We can
> print some warning when lag is higher than latency instead?

Hmmm... what is important is when the lag changes values.

Generally one would indeed expect that to be smaller than the latency, but
that is not really possible when transaction are very fast, say under "-S"
with read-only queries that hit the memory cache.

Also the problem with printing warnings is that it changes the output
format, but it seems to me more useful to print the value, so that it can
be processed automatically and simply.

Also, from a remote client perspective, say a web application, the overall
latency is the lag plus the transaction latency: you first wait to get
through the database (lag), and then you can perform your transaction
(latency).

> Or we can use this value, but it should be better documented, please.

Is the documentation pointed above enough?

--
Fabien.