PING: RFR(m): 8220762: Rework EventLog message string handling and provide VM.events command

[dean.long at oracle.com]

Hi Thomas.  Have you considered breaking this up into three parts: 
variable-sized, jmd, and removing templates?  It seems like they can all 
be done independently.

dl


On 5/17/19 9:44 AM, Thomas Stüfe wrote:
> Hi all,
>
> This is a retry - I posted the original RFR in March but got no reaction so
> far.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8220762
> cr:
> http://cr.openjdk.java.net/~stuefe/webrevs/8220762--rewrite-eventlog-system/webrev.00/webrev/
>
> The Eventlog system keeps an array of events which almost completely
> consist of variable sized string messages.
>
> The message buffer consists of fixed-sized records with a fixed-sized char
> array for those messages. This is not a good fit. Mostly, those buffers are
> too large and we waste space. Then, occasionally a message text is too
> large and we truncate. To avoid truncation, stopgap solutions have been
> added in the past (e.g. JDK-8204551) which would just increase the array
> size, therefore avoiding truncation (for now) but increasing memory waste.
>
> A better solution would be to hold the strings in var-length buffers. We do
> not want strdup since that is too expensive and events may be written in
> situations where C heap is flaky.
>
> So we developed a solution where the strings live outside the event records
> in a FIFO buffer of variable length strings - each string just uses exactly
> as much memory as needed, and truncation is not a problem anymore. This
> allows us also to increase the number of event records held.
>
> In addition, we added an explicit jcmd to display events (VM.events) -
> right now the only way to display events is via hs-err file or VM.info,
> which is really cumbersome. The command has a number of options, e.g. it
> allows to filter for one log only or to show entries in reverse order.
>
> Example output of VM.events:
> http://cr.openjdk.java.net/~stuefe/webrevs/8220762--rewrite-eventlog-system/vm.events-example-output.txt
>
> ------
>
> Note that this is not a simple addition, but a rewrite (not a very big one,
> the event log system is not huge). The coding complexity is simplified by
> this patch which I think makes it more maintainable. I did this by
> abandoning the templatized event record idea in favor of a simple record
> structure. The old solution allowed event logs to define their own record
> structure, but in practice this was a feature almost never used. So IMHO it
> makes sense to un-templatize this and reap the benefits of much simpler
> coding.
>
> If you guys think this is not a worthwhile contribution I'll stop hawking
> it. But I think it makes sense.
>
> jdk-submit tests went through, and I saw no issues in our nightlies.
>
> Thanks & Best Regards, Thomas
>
> (Not sure which mailing lists this would go to - obviously hs-runtime, but
> gc log and compiler log are affected too - feel free to redirect)