Proposed changes to the GC logging flags

Michael Finocchiaro michael.finocchiaro at
Fri Apr 4 07:46:17 UTC 2008

That is good idea to make PGCTS the default with PGCD and I also agree that
the TraceClassUnloading is annoying in a gc log.

I would propose that you look at HP's JVM GC log format as something far
easier to parse than the current format.

or perhaps the jstatd format.

After about 5 or 6 years I can almost eyeball the current file format but a
numeric based, space or comma-separated list of the GC cause and the memory
space allocations would be a lot easier for post-mortem diagnosis and
tuning. And, at least in the HP case, the overhead is very minimal (you get
almost the same amount of detail as from -XX:+PrintTenuringDistribution in a
single line)...and since there still is not a robust and easy-to-use
equivalent of HPjmeter (disclosure - I worked for 10 years for HP) for
analyzing the JavaSoft HotSpot JVMs, this would be an improvement.

My 2 cents,

On Thu, Apr 3, 2008 at 11:22 PM, Tony Printezis <tony.printezis at>

> We're proposing a few small changes (and on significant one) to the way
> the GC logging flags work:
> a) When +PrintGCDetails is set, +PrintGCTimeStamps should be set by
> default (currently, it has to be set explicitly; what we've found is
> that it's always more useful to have the time stamps in a GC log when
> analyzing it)
> b) When -XXloggc: is set, +PrintGCDetails and +PrintGCTimeStamps are set
> automatically (instead of -verbosegc and +PrintGCTimeStamps) (again, for
> us, it's always more useful to have the more detailed +PrintGCDetails
> output in the log when analyzing it)
> c) When -verbosegc, +PrintGCDetails, or -Xloggc: are set,
> +TraceClassUnloading is _not_ automatically set, as it is now (it
> polutes the output a bit during Full GCs)
> Notice that, for the above, you'll still be able to get the old behavior
> by setting / unsetting some flags; we're just trying to set more
> sensible defaults.
> The more significant proposal:
> d) Eliminate the -verbosegc format in favor of the (more detailed and
> more useful, though less concise) +PrintGCDetails format. Do people
> still heavily use and rely on the -verbosegc format in a way that
> migration to the +PrintGCDetails format will be difficult?
> Your thoughts and feedback on the above will be appreciated. Thank you,
> Tony, HS GC Group
> --
> ----------------------------------------------------------------------
> | Tony Printezis, Staff Engineer    | Sun Microsystems Inc.          |
> |                                   | MS BUR02-311                   |
> | e-mail: tony.printezis at    | 35 Network Drive               |
> | office: +1 781 442 0998 (x20998)  | Burlington, MA01803-0902, USA  |
> ----------------------------------------------------------------------
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Michael Finocchiaro
michael.finocchiaro at
Mobile Telephone: +33 6 67 90 64 39
MSN: le_fino at
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