Cassandra 5 JDK21 Command Line actions

[charlie hunt]

Hi Vivekanand,

If you have not already found the general suggestions for tuning ZGC, 
you can find those here:
https://docs.oracle.com/en/java/javase/21/gctuning/z-garbage-collector.html#GUID-8637B158-4F35-4E2D-8E7B-9DAEF15BB3CD

One of the more important parts on that page that is often missed when 
using transparent huge pages is not only setting transparent huge pages 
"enabled = madvise", but also setting "shmem_enabled=advise". I also 
tend to also set transparent huge pages "defrag=madvise" too.

hths,

Charlie

On 5/9/25 4:00 AM, Stefan Johansson wrote:
> 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.
>