Re: [PATCH 2/3] perf events: Introduce realtime clock event

On 02/18/11 07:58, Peter Zijlstra wrote:

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I'm really not sure why you want CLOCK_REALTIME and I think
CLOCK_MONOTONIC is more useful (I'd argue you want your system logs to
contain both, every admin who's ever had to untangle what happened
during DST switches will agree)

I believe CLOCK_MONOTONIC is what perf_clock is tied to -- the
timestamps for PERF_SAMPLE_TIME -- so we already have that.

Its not (it mere _can_ be), it could be tied to the TSC which can
significantly drift wrt CLOCK_MONOTONIC.

Ok, either way I would like correlation between perf_clock and the time
sample data and gettimeofday.

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Programs that generate time-of-day output are using gettimeofday which
is tied to CLOCK_REALTIME. We want to be able to correlate a perf sample
to an entry in an applications log file.

Well, you can argue those programs are broken :-), Imagine the joys of
trying to figure out wth happens when DST jumps the clock back an hour
and you have an hour of duplicate data.

Luckily DST only happens twice a year. Of course reboots happen a little
more often and those reset the monotonic clock.

I can't change the known universe of programs that create pretty
HH:MM:SS MM/DD/YY time strings. What I do want is to know why a program
missed a heartbeat as noted by a log entry. Correlating with a perf
event and seeing the backtrace is quite handy.

David