Advance notice of an imminent Full GC

[Tony Printezis]

On Tue, 8 May 2007, Denis Baranov wrote:

Denis,

(apologies for the late e-mail)

So, it was me who suggested this. To give a bit of background: it's 
probably not possible to notify a user through JMX that "be careful, we 
are about to do a full GC". When we decide to do a full GC, on many 
occasions, we are already in a stop-the-world pause and we typically carry 
on and do a full GC. So, any Java-level code would have no chance in 
actually getting the notification before the full GC completes.

So, I think the best alternative is to just publish a probability of how 
likely a full GC is. Then it's up to the user to decide how conservative 
they want to be when handling this probability. I _think_ such a 
probability will be quite easy to calculate with the statistics we 
maintain in our GCs (but, maybe, we do not make enough of them public in 
order for the user to be able to calculate it?).

Thoughts / suggestions would be appreciated.

Tony

> Hi!
>
> I was told at the JavaOne booth that it may be possible to expose
> HotSpot's inner workings to the degree that when HotSpot makes the
> decision to collect or not, those values it bases its decision upon
> can be exposed via JMX and some application may be notified when
> certain threshold is crossed. I think this would be helpful (in
> absence of RealTime JVM) to be able to notify load balancer in
> load-balanced configuration to stop sending requests to this instance,
> force GC or wait for it to occur naturally, and then notify load
> balancer again that the instance is ready to process requests.
> Otherwise we observe situation when requests pile up at the "gate" and
> cause an abrupt spike in load after Full GC.
>
> Thanks!
>

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| Tony Printezis, Staff Engineer, Java SE | Sun Microsystems Inc.         |
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