RFR: 8258431: Provide a JFR event with live set size estimate [v12]

[Thomas Schatzl]

On Wed, 17 Mar 2021 17:19:06 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Still I think there might be need for the raw data if available, and if it's easily obtainable, then fine, do send some duplicate data. So my suggestion and what we in the gc team can support is to
>
> a) provide that HeapSummary event with capacity() and used() (but as mentioned, on a change they are sent out already so I do not see the exact situation in particular with continuous tracking...).

There has been some question in the latter part of this statement: the "but as mentioned...." part refers to the situation that if you are already continuously monitoring, you will get all of the `used()` events the VM currently sends anyway (if subscribed). So this periodic event does not give you more information.

There may be need for sending `used()` in particular more often as it is done now (and I am open to somebody improving this), not for convenience but because something interesting happens in the Java heap. I am not sure that a "JFR chunk" is the right periodicity though, because it can potentially mean sending (assuming that a "chunk" is some fixed amount of events) every ms to every hour or day. This "random" sampling may send events both too often and too infrequent and not when it matters.

Thomas

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PR: https://git.openjdk.java.net/jdk/pull/2579