The initial question from Will was that we were not consistent in
setting the minimum values. He was also
concerned about the system hanging if the value was too low. As my
experiment shows, the minimum
values are sufficiently high to prevent the system from hanging, i.e.
they are safe. It seemed to us that
Will was also concerned about the performance impact. Clearly from the
measurements, if the count
value is too low the tool will incure an excessive amount of overhead.
The Oprofile documenation says
that Oprofile has less then a 5% performance impact (Ch 6). In chapter
3.6, it warnes about making the
count too small and incurring excessive overhead. The thought behind
raising the minimums was to
ensure the tool did not incure an excessive performance impact, i.e.
keep it no more then 5%. We looked
at several other systems and note that the min count for clocks is at
100,000 which I suspect is high enough
not only to avoid system hangs but also keep the overhead very low.
Since Oprofile uses the minimum
count if no value is specified, it seems like you would want a value
that would give reasonable performance.
The idea behind the change was to be consistent between the IBM
platforms and be consistent with what has
been done on the other platforms.
So, the question to you and Will is, what are the guidelines to setting
the minimum values?
We are willing to change them for 970, power 4 and power 5 to make them
consistent across the various platforms,
if the two of you desire that. We have data that shows what the
settings need to be to ensure the system runs and
what the setting should be to keep the overhead at a reasonable level.
We will be happy to do make any
changes to the minimum values that you and Will feel that is an
appropriate thing to do.
Thanks for the input.
John Levon wrote:
On Thu, Oct 14, 2004 at 04:28:30PM -0500, Maynard P. Johnson wrote:
Carl Love wrote:
Based on the above data, we should set instructions and clocks to a
minimum of 50,000.
The rest of the events can be set at 1000 for a minimum
Maynard is updating the minimum counts for the 970, Power 4 and Power 5
event files. He
will resend the entire 970 patch. In seperate messages, he will send
patches to update the
Power 4 and Power 5 event files.
Attached is a re-send of the full ppc970 patch, with the event file
updated as described above.
I wasn't clear on this. Are you setting new minimums because the old
ones were *unsafe*, or because they just had high overhead? Overhead is
not a good reason to set a minimum...