Will ZGC Allocation Stall block the whole application during the whole GC phase in JDK17?

[Stefan Johansson]

Hi Roy,

On 2024-08-27 11:44, Roy Zhang wrote:
> Dear ZGC experts,
> 
> We are using ZGC on JDK17 (AWS JDK17, OpenJDK 64-Bit Server VM 
> (17.0.10+8-LTS) for linux-amd64 JRE (17.0.10+8-LTS), built on Feb  6 
> 2024 19:58:14 by "jenkins" with gcc 7.3.1 20180303 (Red Hat 7.3.1-5)), 
> from JFR, we found there are several consecutive Allocation Stall event, 
> high GC duration (8s+) but low pause(<< 1ms), and our health check 
> failed (2s timeout) during that period, no safepoint begin(default 10ms 
> threshold) JFR event.
> 

The events you shared are the Garbage Collection JFR events following an 
allocation stall, not the stalls themselves. There is a specific event 
named jdk.ZAllocationStall that, if enabled, provide more information 
about the actual allocation stalls. Those will show you what threads are 
being stalled and for how long.

> I am curious, will ZGC allocation stall  block the application to serve 
> health check requests during the whole phase (8s+)?  Otherwise I can't 
> understand why the health check will fail?
> 

The allocation stalls are per thread, so when an application threads 
needs to allocate and it can't be satisfied it will stall. This will 
also kick off a garbage collection with the "Allocation Stall" cause 
(the ones you see). Other threads can continue to run until they need to 
do an allocation that can't be satisfied (which hopefully can be avoided 
by the GC freeing up memory).

You should also keep in mind that when running into allocation stalls, 
ZGC is not able to fully keep up with the allocation pressure. You 
should look at tuning your instance to avoid this. You can try 
increasing the heap size or allow ZGC to use more concurrent GC threads. 
An even better alternative, if possible, would be to try out JDK 21 and 
use generational ZGC (-XX:+ZGenerational). The generational version of 
ZGC can usually handle high allocation pressure better than single 
generational ZGC.

Hope this helps,
StefanJ

> Really appreciate ur great help in advance!
> 
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> Thanks
> Roy