Cassandra 5 JDK21 Command Line actions

[Stefan Johansson]

Hi Vivekanand,

I'm actually currently trying to figure out how much information I can 
share around the setup we used. I can summarize what is already public 
information from my presentations.

I'm using Cassandra 4 and JDK 21, with tweaked command-line options 
(--add-opens and so) to get it to run properly. I try to do very minimal 
JVM/GC tuning apart from setting the heap size and enabling the use of 
NUMA and large pages. G1 is using a 31G fixed heap (-Xmx31g -Xms31g) to 
be able to make use of compressed oops, while ZGC is using a 32G fixed 
heap. The only GC tuning I do is setting a 50ms pause time goal 
(-XX:MaxGCPauseMillis=50) for one of the G1 runs to see how lowering the 
pause target affect the performance.

The testing methodology is running the same scenario with more and more 
threads to add more and more pressure to the server instance. The server 
is running on its own host and it is a single node setup. The clients 
all run on a different machine and we use the cassandra-stress tool with 
more and more threads to generate the load.

Regarding tuning, one of the big things with ZGC is that you generally 
should not have to do any tuning. That said, when using JDK 21 you need 
to supply the -XX:+ZGenerational option to enable the generational mode. 
In JDK 23 and later the generational mode is on by default and in JDK 24 
ZGC is generational and it can't be turned off.

Something to look out for when using ZGC is allocation stalls, those 
happen when ZGC can't collect and free up memory fast enough. Generally 
if you see allocation stalls, you need to increase the heap size to give 
ZGC more head room to complete the GC work in time. If the stalls happen 
due to unexpected spikes of allocations the SoftMaxHeapSize option can 
be used to set the heap size ZGC should aim at using, but can go over 
during spikes of allocations. For example -Xmx32g 
-XX:SoftMaxHeapSize=28g, would give ZGC 4g of "reserved head room" that 
will only be used when it can't keep up.

I hope this helps,
StefanJ

On 2025-05-09 07:27, Vivekanand Koya wrote:
> Hello ZGC members,
>
> I work on the Apache Cassandra team implementing support for JDK21 in 
> the upcoming Cassandra 5+ release. I need insight into JVM options 
> providing comparable and perhaps improved performance compared to 
> G1GC. Some work has been done using the previous defaults: 
> https://github.com/apache/cassandra/commit/b15d4d6980e787ab5f3405ca8cb17a9c92a4aa47. 
> Also, can you please provide the testing/benchmarking methodology used 
> by Stefan Johansson when presenting at Devoxx.
>
> Hope to achieve greater outcomes together,
> Thanks,
> Vivekanand K.
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