A more honest comparison would be using a Executors.newCachedThreadPool() so it
is also unlimited connections, or using a Semaphore with 4 permits around the
virtual
Regards
On Wed, Nov 26, 2025, at 11:32, David wrote:
> Hi,
>
> There seems to be a limit causing a 12-30 second delay after establishing ~8K
> connections too quickly. 10K platform threads do not see or reach this limit.
>
> The simplest workaround I found was adding System.out.println("1"); right
> before making each request. The synchronized lock is probably doing the heavy
> lifting here. This makes the platform and virtual threads perform about the
> same.
>
> Increasing the server-side Tomcat settings also resolves the stalling:
> • server.tomcat.accept-count=10000
> • server.tomcat.max-connections=20000
> • server.tomcat.threads.max=500
> • server.tomcat.threads.min-spare=50
> However, with these higher server limits, I'm seeing different throughput
> characteristics:
> • Platform threads: 2.3 ops/s
> • Virtual threads: 0.9 ops/s (about half the speed)
>
>> I assume it also benefits from being able to use HTTP persistent
>> connections.
>
> Just trying to understand, does that mean that virtual threads don't have
> this? Because running Socket Statistics in the background showing 17K
> established connections while running virtual threads vs 140 when using
> platform threads.
>
> At this point I am just trying to understand where this delay is coming from
> and why virtual threads trigger this.
>
> Thanks in advance!
>
> Kind regards,
> David
>
>
>
> On Wed, 26 Nov 2025 at 10:33, Alan Bateman <[email protected]> wrote:
>>
>>
>> On 26/11/2025 00:25, Robert Engels wrote:
>> > Your platform test is limiting to at most 4 outstanding requests.
>>
>> I assume it also benefits from being able to use HTTP persistent
>> connections.
>>
>> The benchmark using virtual threads is very different, it tries to
>> establish 10k connections in a burst. Do you know what connection
>> backlog is used by Tomcat? It may require adjusting net.core.somaxconn
>> (kern.ipc.somaxconn on macOS) and other settings. ~8100 may be 8k and
>> maybe there is something in the Tomcat or Spring connection that
>> controls this.
>>
>> -Alan
