Performance Impact of Systemtap

[Frank Ch. Eigler]

On Wed, Sep 07, 2011 at 08:42:23PM -0600, ydev wrote:
> Thank you Frank. I'm very new to SystemTap so could you please
> tell me what is the -DSTP_ALIBI mode run.

-DSTP_ALIBI compiles out the probe handlers from the systemtap module,
leaving only the parts that hook into the kernel, and just keep hit
counts/time.  So it's as if the probe handlers were all { } empty.

> [...] I started systemtap in flight mode and triggered many
> reads/write and used the 'time' command to measure how long it takes
> to execute cat.

OK.

> I did this without systemtap running in the background. The figures
> were almost the same. I'm not sure if using 'time' command gives me
> accurate results.

Overall elapsed times should be accurate with /bin/time.
Instrumenting relatively few & slow calls like disk I/O should indeed
have negligible overhead when instrumented with systemtap.

- FChE