Interesting profile session

[robert engels]

Will do and I’ll report back. 

> On Jun 25, 2024, at 3:14 AM, Alan Bateman <Alan.Bateman at oracle.com> wrote:
> 
>  On 24/06/2024 23:03, Robert Engels wrote:
>> 
>> The VT latency is a couple of orders of magnitude higher than the platform latency.
>> 
>> I suspect that what is happening is that the virtual threads are parking waiting for work too soon - or that the read & writes are always causing a context switch - regardless of if the call is very short (room in the kernel socket buffers).
>> 
>> Can you suggest any remedies? It feels like the scheduler needs some heuristics on average call time to determine and a spin loop to avoid this sort of thing.
>> 
> There may be a combination of things here. From the stack traces, this looks like JDK 21 + macOS. There has been significant changes to both the scheduler (ForkJoinPool) and the I/O poller mechanism since then.  Would you have time to out the latest EA build [1] to see if you get similar latency? It would be interesting to see data with -Djdk.pollerMode=1 and 2 (the default on macOS is "a"). Also would be useful to see `jcmd <pid> Thread.vthread_summary` output while the benchmark is running as that will reveal more about the scheduler and how many virtual threads are blocked on socket I/O.
> 
> [1] https://jdk.java.net/loom/
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