examples/vsock-latency: reproducer for first-connection vsock delay#683
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examples/vsock-latency: reproducer for first-connection vsock delay#683smoser wants to merge 2 commits into
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The first host-to-guest vsock round trip after VM start blocks until
libkrun's vsock reaper TTL fires (src/devices/src/virtio/vsock/reaper.rs:10,
currently 5 s). The first host connect() succeeds at the unix layer
immediately, but the first read() blocks for the full reaper window
before libkrun closes the unix socket with EOF. A second connect()
immediately afterwards uses a fresh proxy and completes in <1 ms.
The reproducer is ~400 lines of C in a new examples/vsock-latency/
subdirectory:
* guest.c tiny static AF_VSOCK listener; one ack byte per connect
* host.c libkrun setup + a timing thread that prints
phase: socket-created +<t>
phase: first-roundtrip-ok +<t> (attempts=<n>)
warm-roundtrip[0..3]: +<t>
* Makefile builds both; "make rootfs" pulls a busybox image
into ./rootfs/ via docker (a one-file rootfs is not
enough for libkrunfw's init)
* README.md what it measures, expected output, root cause
The host.c timing loop also honours PROBE_TIMEOUT_MS to apply
SO_RCVTIMEO per attempt (simulates a caller that wraps each probe
in a deadline). Measured trade-off on libkrunfw 5.4.0 / bundled
kernel 6.12.87:
PROBE_TIMEOUT_MS first-roundtrip-ok attempts
unset ~5340 ms 2
2000 ~2355 ms 2
100 ~425 ms 2
10 ~373 ms 3
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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This was opened to share a reproducer for #684