Am 24.03.2023 um 11:47 hat Florian Westphal geschrieben: > 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);
In the error paths, we never call set_cork(false) again. I suppose the reason that this is okay is because the next thing is actually that we close the socket? Kevin