qemu-nbd doesn't set TCP_NODELAY on the tcp socket.
Kernel waits for more data and avoids transmission of small packets.
Without TLS this is barely noticeable, but with TLS this really shows.
Booting a VM via qemu-nbd on localhost (with tls) takes more than
2 minutes on my system. tcpdump shows frequent wait periods, where no
packets get sent for a 40ms period.
Add explicit (un)corking when processing (and responding to) requests.
"TCP_CORK, &zero" after earlier "CORK, &one" will flush pending data.
VM Boot time:
main: no tls: 23s, with tls: 2m45s
patched: no tls: 14s, with tls: 15s
VM Boot time, qemu-nbd via network (same lan):
main: no tls: 18s, with tls: 1m50s
patched: no tls: 17s, with tls: 18s
Future optimization: if we could detect if there is another pending
request we could defer the uncork operation because more data would be
appended.
Signed-off-by: Florian Westphal <f...@strlen.de>
---
nbd/server.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/nbd/server.c b/nbd/server.c
index a4750e41880a..848836d41405 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -2667,6 +2667,8 @@ static coroutine_fn void nbd_trip(void *opaque)
goto disconnect;
}
+ qio_channel_set_cork(client->ioc, true);
+
if (ret < 0) {
/* It wasn't -EIO, so, according to nbd_co_receive_request()
* semantics, we should return the error to the client. */
@@ -2692,6 +2694,7 @@ static coroutine_fn void nbd_trip(void *opaque)
goto disconnect;
}
+ qio_channel_set_cork(client->ioc, false);
done:
nbd_request_put(req);
nbd_client_put(client);
--
2.39.2
