Re: [PATCH 4/8] udp: Receive multiple datagrams at once on the pasta sock->tap path

[Stefano Brivio <sbrivio@redhat.com>]

Sorry for the further delay,

On Wed, 14 Dec 2022 11:35:46 +0100
Stefano Brivio <sbrivio@redhat.com> wrote:

> On Wed, 14 Dec 2022 12:42:14 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Tue, Dec 13, 2022 at 11:48:47PM +0100, Stefano Brivio wrote:  
> > > Sorry for the long delay here,
> > > 
> > > On Mon,  5 Dec 2022 19:14:21 +1100
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > >     
> > > > Usually udp_sock_handler() will receive and forward multiple (up to 32)
> > > > datagrams in udp_sock_handler(), then forward them all to the tap
> > > > interface.  For unclear reasons, though, when in pasta mode we will only
> > > > receive and forward a single datagram at a time.  Change it to receive
> > > > multiple datagrams at once, like the other paths.    
> > > 
> > > This is explained in the commit message of 6c931118643c ("tcp, udp:
> > > Receive batching doesn't pay off when writing single frames to tap").
> > > 
> > > I think it's worth re-checking the throughput now as this path is a bit
> > > different, but unfortunately I didn't include this in the "perf" tests :(
> > > because at the time I introduced those I wasn't sure it even made sense to
> > > have traffic from the same host being directed to the tap device.
> > > 
> > > The iperf3 runs were I observed this are actually the ones from the Podman
> > > demo. Ideally that case should be also checked in the perf/pasta_udp tests.    
> > 
> > Hm, ok.
> >   
> > > How fundamental is this for the rest of the series? I couldn't find any
> > > actual dependency on this but I might be missing something.    
> > 
> > So the issue is that prior to this change in pasta we receive multiple
> > frames at once on the splice path, but one frame at a time on the tap
> > path.  By the end of this series we can't do that any more, because we
> > don't know before the recvmmsg() which one we'll be doing.  
> 
> Oh, right, I see. Then let me add this path to the perf/pasta_udp test
> and check how relevant this is now, I'll get back to you in a bit.

I was checking the wrong path. With this:

--
diff --git a/test/perf/pasta_udp b/test/perf/pasta_udp
index 27ea724..973c2f4 100644
--- a/test/perf/pasta_udp
+++ b/test/perf/pasta_udp
@@ -31,6 +31,14 @@ report	pasta lo_udp 1 __FREQ__
 
 th	MTU 1500B 4000B 16384B 65535B
 
+tr	UDP throughput over IPv6: host to ns
+nsout	IFNAME ip -j link show | jq -rM '.[] | select(.link_type == "ether").ifname'
+nsout	ADDR6 ip -j -6 addr show|jq -rM '.[] | select(.ifname == "__IFNAME__").addr_info[] | select(.scope == "global" and .prefixlen == 64).local'
+bw	-
+bw	-
+bw	-
+iperf3	BW host ns __ADDR6__ 100${i}2 __THREADS__ __TIME__ __OPTS__ -b 15G
+bw	__BW__ 7.0 9.0
 
 tr	UDP throughput over IPv6: ns to host
 ns	ip link set dev lo mtu 1500
diff --git a/test/run b/test/run
index e07513f..b53182b 100755
--- a/test/run
+++ b/test/run
@@ -67,6 +67,14 @@ run() {
 	test build/clang_tidy
 	teardown build
 
+	VALGRIND=0
+	setup passt_in_ns
+	test passt/ndp
+	test passt/dhcp
+	test perf/pasta_udp
+	test passt_in_ns/shutdown
+	teardown passt_in_ns
+
 	setup pasta
 	test pasta/ndp
 	test pasta/dhcp
--

I get 21.6 gbps after this series, and 29.7 gbps before -- it's quite
significant.

And there's nothing strange in perf's output, really, the distribution
of overhead per functions is pretty much the same, but writing multiple
messages to the tap device just takes more cycles per message compared
to a single message.

I'm a bit ashamed to propose this, but do you think about something
like:

	if (c->mode == MODE_PASTA) {
		if (recvmmsg(ref.r.s, mmh_recv, 1, 0, NULL) <= 0)
			return;

		if (udp_mmh_splice_port(v6, mmh_recv)) {
			n = recvmmsg(ref.r.s, mmh_recv + 1,
				     UDP_MAX_FRAMES - 1, 0, NULL);
		}

		if (n > 0)
			n++;
		else
			n = 1;
	} else {
		n = recvmmsg(ref.r.s, mmh_recv, UDP_MAX_FRAMES, 0, NULL);
		if (n <= 0)
			return;
	}

? Other than the inherent ugliness, it looks like a good approximation
to me.

-- 
@@ -67,6 +67,14 @@ run() {
 	test build/clang_tidy
 	teardown build
 
+	VALGRIND=0
+	setup passt_in_ns
+	test passt/ndp
+	test passt/dhcp
+	test perf/pasta_udp
+	test passt_in_ns/shutdown
+	teardown passt_in_ns
+
 	setup pasta
 	test pasta/ndp
 	test pasta/dhcp
--

I get 21.6 gbps after this series, and 29.7 gbps before -- it's quite
significant.

And there's nothing strange in perf's output, really, the distribution
of overhead per functions is pretty much the same, but writing multiple
messages to the tap device just takes more cycles per message compared
to a single message.

I'm a bit ashamed to propose this, but do you think about something
like:

	if (c->mode == MODE_PASTA) {
		if (recvmmsg(ref.r.s, mmh_recv, 1, 0, NULL) <= 0)
			return;

		if (udp_mmh_splice_port(v6, mmh_recv)) {
			n = recvmmsg(ref.r.s, mmh_recv + 1,
				     UDP_MAX_FRAMES - 1, 0, NULL);
		}

		if (n > 0)
			n++;
		else
			n = 1;
	} else {
		n = recvmmsg(ref.r.s, mmh_recv, UDP_MAX_FRAMES, 0, NULL);
		if (n <= 0)
			return;
	}

? Other than the inherent ugliness, it looks like a good approximation
to me.

-- 
Stefano