On 11/02/2026 16:00, Matthew Swift wrote:
Hi,
I suspect that I've discovered a bug in the IO polling mechanism used
by virtual threads on Windows. I've been able to systematically
reproduce the problem on Windows 2016, 2019 and 2022 Intel x64 using
both JDK 25.0.2 and 25.0.3 EA build 01, and also JDK 27 EA 8 build.
The problem occurs when the server attempts to send a large response
message to the client. The server's write operation becomes blocked
and is never notified when the socket becomes writable again:
Can you determine if a blocking read outstanding on the same connection
that another thread is blocked on the large write? Can you set SO_SNDBUF
to a large value to see if that reduces or eliminates the sightings?
We have been chasing an issue in the Windows Ancillary Function Driver
for winsock that arises when a Windows SOCKET is use with two different
completion ports or AFD device handles at the same time. Daniel Jeliński
has some good analysis here:
https://github.com/piscisaureus/wepoll/issues/35
We have a prototype poller implementation that works around this issue
but it has some performance impact (on Windows) so have been slow to
process it [1].
-Alan
[1] https://bugs.openjdk.org/browse/JDK-8334574
