RFR(XS): 7143858: G1: Back to back young GCs with the second GC having a minimally sized eden

[Bengt Rutisson]

John,

Looks good!

Nice catch.

Bengt

22 maj 2012 kl. 01:12 skrev John Cuthbertson <john.cuthbertson at oracle.com>:

> Hi Everyone,
> 
> Can I have a couple of volunteers review the fix for this CR? The webrev can be found at: http://cr.openjdk.java.net/~johnc/7143858/webrev.0/
> 
> Description:
> 
> In some recent G1 logs we have seen some evacuation pauses that looked premature, i.e. the eden occupancy was much less than the target capacity - usually only one or two regions. These premature pauses always (almost immediately) followed a normal evacuation pause with little or no application activity in-between. Bengt's recent changes to display the GC cause indicated that the premature pauses were always GCLocker Initiated GCs.
> 
> What seemed to have been happening was that as the last left a JNI critical region, before it scheduled the GCLocker Initiated GC, another thread would attempt an allocation. The allocating thread would see that GCLocker was no longer active and successfully schedule the evacuation pause. As part of this GC operation, a mutator alloc region would get allocated and the object allocation request would be satisfied. After, the thread initating the GCLocker GC would schedule its GC and the eden occupancy would be fairly minimal.
> 
> Inserting a 500ms sleep just before scheduling the GCLocker initated GC was able to reproduce the problem with Dacapo fairly frequently.
> 
> The solution implemented in this webrev is to stall the allocating thread until the GCLocker Initiated GC is performed and then retry the allocation.
> 
> Testing: GC Test Suite and Kitchensink with the additional sleep call and verify that no pauses occurred during the sleep, GC Test suite and jprt with the additional sleep.
> 
> I have been unable to reproduce the issue with the other Hotspot collectors but I can't see why they wouldn't be vulnerable. I changed the G1 slow case allocation code to match the other collectors and still saw the issue.
> 
> Thanks,
> 
> JohnC
> 
> 
>