Comments (6)
Hi Pether,
First, importance of max values is often over-estimated by managers in my
experience: what they see is a fact that some users have long waits sometimes,
but in fact if the long waits are rare (one among thousands hits) these long
waits may have gone almost unnoticed and without much importance.
That's why there is a "standard deviation" number next to the "max time" in the
report: if the standard deviation is low relative to the max time, then long
waits are rare. And it is the case for you here: standard deviations are 42 / 6
for 156 298 ms / 169 511 ms max times. So these max times seems very rare,
perhaps one in a million executions.
Second, web servers and java in particular are mostly based on best effort (you
are certainly not using real time java and moreover your are using a database).
And so waits can happen sometimes for thousands of reasons, among which many
are out of reach for the java guy in the shop. Lowering waits can be easy at
first, but lowering long waits below one in a million executions can be very
very expensive (we do not put hard real time java everywhere for a good
reason). For example, a major GC or some memory/disk access errors or a backup
of the database can happen sometimes: one life critical application which I was
involved with was quite slow some days, until we discovered later that desktop
admins have scheduled a defragmentation of hard disks every Friday for all
desktops in the enterprise.
So these high numbers are rare occurences in your case and their importance
should really not be overestimated, but we could try to explain them:
- in javamelody, the max time "156 298 ms" is not the max for "exec
sp_session_starta 60" when executed by SchemaSignatur.jsp. It is the max time
for all the "exec sp_session_starta 60" executions in your application.
Perhaps, this sql query was used for the selected period elsewhere in your
application: you can try to find where by opening the detail for "exec
sp_session_starta 60" and then by clicking "find usages". And this could
explain why max times are high for this sql but not so high for this jsp.
- for the same reason, a long execution can be recorded and displayed for the
sql but not yet for the jsp. For example, long sql executions can be recorded
and for whatever reason the server is killed, the jsp execution will never end
and be recorded.
- then these max times can be real or they can be a false measure. If it is
real, there are thousands of reasons for that in the java server or in the
database (remember: one in a million).
- and duration are based on wall clock time, so a synchronization of the clock
in the OS between the begin and the end of the sql can introduced rare false
measures: the measured time can be very long or it can be negative (but
negatives are changed to 0 by javamelody). Synchronisation of clock time is
enabled by default in Windows.
(- and hopefully an error in javamelody computations can't happen)
So your case is in fact possible, but probably not important given your
standard deviations.
Original comment by [email protected]
on 29 Sep 2010 at 8:54
from javamelody.
Thanks for the rapid response. Understand now more about the standard
deviations and might been possible that the server was shutdown during some of
the request.
Original comment by [email protected]
on 29 Sep 2010 at 6:25
from javamelody.
ok, I suppose that it was a shutdown during a request.
(please reopen if needed)
bye
Original comment by [email protected]
on 4 Oct 2010 at 7:59
- Changed state: Invalid
from javamelody.
Original comment by [email protected]
on 9 Nov 2010 at 7:04
- Changed state: Accepted
from javamelody.
Original comment by [email protected]
on 9 Nov 2010 at 7:05
from javamelody.
Reasons above are valid, but still there was on maximum values in statistics as
said in issue 59, which is now fixed.
So setting this issue as fixed as the possible reason of it was finally the
issue 59 and the executions of days and weeks before.
Thanks
Original comment by [email protected]
on 9 Nov 2010 at 7:12
- Changed state: Fixed
from javamelody.
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