Re: Performance glitch in GetCurrentAbsoluteTime()

(back online after a week of downtime)

> In utils/adt/nabstime.c, the function GetCurrentAbsoluteTime() is called
> during each StartTransaction in order to save the transaction's start
> time. It shows up unreasonably high in my profile (> 1% of runtime):
> 0.62 10.22 100001/100001 StartTransaction [65]
> [91] 1.4 0.62 10.22 100001 GetCurrentAbsoluteTime [91]
> 0.92 8.30 100001/100001 localtime [105]
> 0.88 0.00 100001/100004 time [305]
> 0.12 0.00 100001/104713 strcpy [479]
> Now the interesting thing about this is that the essential part of the
> function is just the time() call, AFAICS, and that's quite cheap. More
> than 90% of the runtime is being spent in the "if (!HasCTZSet)" branch.
> I see no reason for that code to be run during every single transaction.
> It sets the following variables:
> CTimeZone
> CDayLight
> CTZName
> CDayLight is not used *anywhere* except for debug printouts, and could
> go away completely.

OK, let's kill it.

> CTZName is not used if USE_POSIX_TIME is defined,
> which is true on most platforms.

OK, it should be #ifndef'd

> CTimeZone is not quite as useless, but
> there are only a couple places where it's used when USE_POSIX_TIME is
> true, and they don't look like critical-path stuff to me.
> We could almost say that these variables need only be set once per
> backend startup, but I suppose that would do the wrong thing in a
> backend that's left running over a daylight-savings transition.

Right. If we were only supporting WinDoze, then we wouldn't need to
worry. But my linux box stays up forever, so daylight savings time
transitions are important ;)

> What I'm inclined to do is arrange for these variables to be calculated
> only on-demand, at most once per transaction. It'd be even nicer to
> get rid of them entirely, but I don't think I understand the time code
> well enough to venture that.

At most once per transaction is what I was hoping the behavior already
is. Anyway, if we can take the time() result and *later* figure out
the other values, then we could:

1) clear a flag when time() is called
2) use a wrapper around a stripped GetCurrentAbsoluteTime() for
date/time support
3) if the flag in (1) is clear, then evaluate the other parameters

- Thomas

--
Thomas Lockhart lockhart(at)alumni(dot)caltech(dot)edu
South Pasadena, California