Re: [Proposal] Add accumulated statistics for wait event

Michael Paquier <michael@paquier.xyz> writes:
> This does not need a configure switch.

It probably is there because the OP realizes that most people wouldn't
accept having this code compiled in.

> What's the performance penalty?  I am pretty sure that this is
> measurable as wait events are stored for a backend for each I/O
> operation as well, and you are calling a C routine within an inlined
> function which is designed to be light-weight, doing only a four-byte
> atomic operation.

On machines with slow gettimeofday(), I suspect the cost of this
patch would be staggering.  Even with relatively fast gettimeofday,
it doesn't look acceptable for calls in hot code paths (for instance,
lwlock.c).

A bigger problem is that it breaks stuff.  There are countless
calls to pgstat_report_wait_start/pgstat_report_wait_end that
assume they have no side-effects (for example, on errno) and
can never fail.  I wouldn't trust GetCurrentTimestamp() for either.
If the report_wait calls can't be dropped into code with *complete*
certainty that they're safe, that's a big cost.

Why exactly is this insisting on logging timestamps and not,
say, just incrementing a counter?  I think doing it like this
is almost certain to end in rejection.

            regards, tom lane