Thread: Performance glitch in GetCurrentAbsoluteTime()
I have been doing some profiling this weekend in response to Vadim's
challenge to reduce the amount of overhead in a simple INSERT command.
I've found a number of simple improvements that I hope to check in
shortly. I came across something in the time code that I thought I'd
better check with you before changing.
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:
CTimeZoneCDayLightCTZName
CDayLight is not used *anywhere* except for debug printouts, and could
go away completely. CTZName is not used if USE_POSIX_TIME is defined,
which is true on most platforms. 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.
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.
Do you have any comments pro or con on this?
regards, tom lane
(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@alumni.caltech.edu
South Pasadena, California
Thomas Lockhart <lockhart@alumni.caltech.edu> writes:
> (back online after a week of downtime)
I was wondering why you were so quiet. Hardware trouble?
>> What I'm inclined to do is arrange for these variables to be calculated
>> only on-demand, at most once per transaction.
> At most once per transaction is what I was hoping the behavior already
> is.
Actually, my gripe is that it's done in every transaction whether
needed or not...
> 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
Right, that was pretty much what I was thinking too. As long as
CTimeZone &etc are evaluated using the time value saved at the
start of the transaction, the behavior will be the same.
regards, tom lane
> > (back online after a week of downtime)
> I was wondering why you were so quiet. Hardware trouble?
In a sense. The server-side modem/ppp setup I use was dead, and I was
so busy on other stuff I didn't bug anyone to fix it...
> Right, that was pretty much what I was thinking too. As long as
> CTimeZone &etc are evaluated using the time value saved at the
> start of the transaction, the behavior will be the same.
Should I put it on my ToDo? Not sure how to test that it helps with
execution time, but I should be able to get to it before 7.0...
- Thomas
--
Thomas Lockhart lockhart@alumni.caltech.edu
South Pasadena, California