Feedback requested: HotSpot GC logging improvements

[Ryan Rawson]

Hey,

I would say that PrintGCDateStamps should be the default - or at least
promoted heavily as the option to use.  Since most other logs (eg:
log4j) are in "normal time" correlating GC logs and server logs
require determining when the VM started, then doing some on the fly
math for every log line you are interested in.  Sometimes determining
VM start time is impossible because the first log line is offset from
the VM start time by X milliseconds, and in a tight debugging
situation this could be all the difference in the world.

On the logfile front, there are at least 2 problems:
- successive runs overwrite the previous log file.  I ran into this
problem and lost the ability to debug a problem.
- a logfile will grow without bound, although in my experience I have
not had space problems with this.

While you are correct that _the cheapest disks_ you can buy run
$85/TB, this is not the kinds of disks many people are installing into
server-type systems. A serial-attached-scsi (aka SAS) disk at 10k rpm
is a little bit more expensive than $85/TB.

-ryan


On Thu, May 6, 2010 at 12:32 PM, Tony Printezis
<tony.printezis at oracle.com> wrote:
> Hi all,
>
> We would like your input on some changes to HotSpot's GC logging that we
> have been discussing. We have been wanting to improve our GC logging for
> some time. However we haven't had the resources to spend on it. We don't
> know when we'll get to it, but we'd still like to get some feedback on
> our plans.
>
> The changes fall into two categories.
>
>
> A. Unification and improvement of -verbosegc / -XX:+PrintGCDetails output.
>
> I strongly believe that maintaining two GC log formats is
> counter-productive, especially given that the current -verbosegc format
> is unhelpful in many ways (i.e., lacks a lot of helpful information).
> So, we would like to unify the two into one, with maybe
> -XX:+PrintGCDetails generating a superset of what -verbosegc would
> generate (so that a parser for the -XX:+PrintGCDetails output will also
> be able to parse the -verbosegc output). The new output will not be what
> -XX:+PrintGCDetails generates today but something that can be reliably
> parsed and it is also reasonably human-readable (so, no xml and no
> space/tab-separated formats). Additionally, we're proposing to enable
> -XX:+PrintGCTimeStamps by default (in fact, we'll probably deprecate and
> ignore that option, I can't believe that users will really not want a
> time stamp per GC log record). We'll leave -XX:+PrintGCDateStamps to be
> optional though.
>
> Specific questions:
>
> - Is anyone really attached to the old -verbosegc output?
> - Would anyone really hate having time stamps by default?
> - I know that a lot of folks have their own parsers for our current GC
> log formats. Would you be happy if we provided you with a (reliable!)
> parser for the new format in Java that you can easily adapt?
>
>
> B. Introducing "cyclic" GC logs.
>
> This is something that a lot of folks have asked for given that they
> were concerned with the GC logs getting very large (a 1TB disk is $85
> these days, but anyway...). Given that each GC log record is of variable
> size, we cannot easily cycle through the log using the same file (I'd
> rather not have to overwrite existing records). Our current proposal is
> for the user to specify a file number N and a size target S for each
> file. For a given GC log -Xloggc:foo, HotSpot will generate
>
> foo.00000001
> foo.00000002
> foo.00000003
> etc.
>
> (we'll create a new file as soon as the size of the one we are writing
> to exceeds S, so each file will be slightly larger than S but it will be
> helpful not to split individual log records between two files)
>
> When we create a new file, if we have more than N files we'll delete the
> oldest. So, in the above example, if N == 3, when we create foo.00000004
> we'll delete foo.00000001.
>
> Note that in the above scheme, the logs are not really "cyclic" but,
> instead, we're pruning the oldest records every now and then, which has
> the same effect.
>
> Another (related) request has been to maybe append the GC log file name
> with the pid of the JVM that's generating it. Maybe we don't want to do
> this by default. But, would people find it helpful if we provide a new
> cmd line parameter to do that? So, for the above example and assuming
> that the JVM's pid is 1234, the GC log file(s) will be either:
>
> foo.1234
>
> or
>
> foo.1234.00000001
> foo.1234.00000002
> foo.1234.00000003
> etc.
>
> Specific questions:
>
> - Would people really hate it if HotSpot starts appending the GC log
> file name with a (zero-padded) sequence number? Maybe if N == 1 (the
> default), HotSpot will skip the sequence number and ignore S, i.e.,
> behave as it does today.
> - To the people who have been asking for cyclic GC logs: is the sequence
> number scheme above good enough?
>
>
> Thanks in advance for your feedback,
>
> Tony, HotSpot GC Group
>
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