Author: tim.bunce
Date: Thu Oct 16 16:07:51 2008
New Revision: 524
Modified:
trunk/lib/Devel/NYTProf.pm
Log:
Point out the perils of profiling on virtual machines.
Modified: trunk/lib/Devel/NYTProf.pm
==============================================================================
--- trunk/lib/Devel/NYTProf.pm (original)
+++ trunk/lib/Devel/NYTProf.pm Thu Oct 16 16:07:51 2008
@@ -273,6 +273,10 @@
happen so rarely relative to the activity of a most applications that you'd
have to run the code for many hours to have any hope of reasonably useful
results.
+(It may be possible to use the C<clock=N> option to select a
+high-resolution cpu time clock. I've not tried that yet.
+If you try it, please let me know how it works out.)
+
=head2 file=...
Specify the output file to write profile data to
(default: './nytprof.out').
@@ -363,6 +367,25 @@
The reporting code currently doesn't handle #line directives, but at least
it
warns about them. Patches welcome.
+
+=head1 CAVEATS
+
+=head2 Virtual Machines
+
+I recommend you don't do performance profiling while running in a
+virtual machine. If you do you're likely to find inexplicable spikes
+of real-time appearing at unreasonable places in your code. You should pay
+less attention to the statement timings and rely more on the subroutine
+timings. They will still be noisy but less so than the statement times.
+
+You could also try using the C<clock=N> option to select a high-resolution
+I<cpu-time> clock instead of a real-time one. That should be much less
+noisy, though you will loose visibility of wait-times due to network
+and disk I/O, for example.
+
+If your system doesn't support the C<clock=N> option then you could try
+using the C<usecputime=1> option. That will give you cpu-time measurements
+but only at a very low 1/00th of a second resolution.
=head1 BUGS
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