On Wed, Jan 04, 2023 at 01:08:52AM +0100, Stefano Brivio wrote: > On Wed, 21 Dec 2022 17:00:24 +1100 > David Gibson wrote: > > > On Tue, Dec 20, 2022 at 11:42:46AM +0100, Stefano Brivio wrote: > > > Sorry for the further delay, > > > > > > On Wed, 14 Dec 2022 11:35:46 +0100 > > > Stefano Brivio wrote: > > > > > > > On Wed, 14 Dec 2022 12:42:14 +1100 > > > > David Gibson 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 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 > > > > Ah, ok. Can we add that to the standard set of tests ASAP, please. > > > > > I get 21.6 gbps after this series, and 29.7 gbps before -- it's quite > > > significant. > > > > Drat. > > > > > 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. > > > > That's so weird. It should be basically an identical set of write()s, > > except that they happen in a batch, rather than a bit spread out. I > > guess it has to be some kind of cache locality thing. I wonder if the > > difference would go away or reverse if we had a way to submit multiple > > frames with a single syscall. > > > > > 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. > > > > Hmm. Well, the first question is how much impact does going 1 message > > at a time have on the spliced throughput. If it's not too bad, then > > we could just always go one at a time for pasta, regardless of > > splicing. And we could even abstract that difference into the tap > > backend with a callback like tap_batch_size(c). > > So, finally I had the chance to try this out. > > First off, baseline with the patch adding the new tests I just sent, > and the series you posted: > > === perf/pasta_udp > > pasta: throughput and latency (local traffic) > Throughput in Gbps, latency in µs, one thread at 3.6 GHz, 4 streams > MTU: | 1500B | 4000B | 16384B | 65535B | > |--------|--------|--------|--------| > UDP throughput over IPv6: ns to host | 4.4 | 8.5 | 19.5 | 23.0 | > UDP RR latency over IPv6: ns to host | - | - | - | 27 | > |--------|--------|--------|--------| > UDP throughput over IPv4: ns to host | 4.3 | 8.8 | 18.5 | 24.4 | > UDP RR latency over IPv4: ns to host | - | - | - | 26 | > |--------|--------|--------|--------| > UDP throughput over IPv6: host to ns | - | - | - | 22.5 | > UDP RR latency over IPv6: host to ns | - | - | - | 30 | > |--------|--------|--------|--------| > UDP throughput over IPv4: host to ns | - | - | - | 24.5 | > UDP RR latency over IPv4: host to ns | - | - | - | 25 | > '--------'--------'--------'--------' > ...passed. > > > pasta: throughput and latency (traffic via tap) > Throughput in Gbps, latency in µs, one thread at 3.6 GHz, 4 streams > MTU: | 1500B | 4000B | 16384B | 65520B | > |--------|--------|--------|--------| > UDP throughput over IPv6: ns to host | 4.4 | 10.4 | 16.0 | 23.4 | > UDP RR latency over IPv6: ns to host | - | - | - | 27 | > |--------|--------|--------|--------| > UDP throughput over IPv4: ns to host | 5.2 | 10.8 | 16.0 | 24.0 | > UDP RR latency over IPv4: ns to host | - | - | - | 28 | > |--------|--------|--------|--------| > UDP throughput over IPv6: host to ns | - | - | - | 21.5 | > UDP RR latency over IPv6: host to ns | - | - | - | 29 | > |--------|--------|--------|--------| > UDP throughput over IPv4: host to ns | - | - | - | 26.3 | > UDP RR latency over IPv4: host to ns | - | - | - | 26 | > '--------'--------'--------'--------' > > which seems to indicate the whole "splicing" thing is pretty much > useless, for UDP (except for that 16 KiB MTU case, but I wonder how > relevant that is). > > If I set UDP_MAX_FRAMES to 1, with a quick workaround for the resulting > warning in udp_tap_send() (single frame to send, hence single message), > it gets somewhat weird: > > === perf/pasta_udp > > pasta: throughput and latency (local traffic) > Throughput in Gbps, latency in µs, one thread at 3.6 GHz, 4 streams > MTU: | 1500B | 4000B | 16384B | 65535B | > |--------|--------|--------|--------| > UDP throughput over IPv6: ns to host | 3.4 | 7.0 | 21.6 | 31.6 | > UDP RR latency over IPv6: ns to host | - | - | - | 30 | > |--------|--------|--------|--------| > UDP throughput over IPv4: ns to host | 3.8 | 7.0 | 22.0 | 32.4 | > UDP RR latency over IPv4: ns to host | - | - | - | 26 | > |--------|--------|--------|--------| > UDP throughput over IPv6: host to ns | - | - | - | 29.3 | > UDP RR latency over IPv6: host to ns | - | - | - | 31 | > |--------|--------|--------|--------| > UDP throughput over IPv4: host to ns | - | - | - | 33.8 | > UDP RR latency over IPv4: host to ns | - | - | - | 25 | > '--------'--------'--------'--------' > ...passed. > > > pasta: throughput and latency (traffic via tap) > Throughput in Gbps, latency in µs, one thread at 3.6 GHz, 4 streams > MTU: | 1500B | 4000B | 16384B | 65520B | > |--------|--------|--------|--------| > UDP throughput over IPv6: ns to host | 4.7 | 10.3 | 16.0 | 24.0 | > UDP RR latency over IPv6: ns to host | - | - | - | 27 | > |--------|--------|--------|--------| > UDP throughput over IPv4: ns to host | 5.6 | 11.4 | 16.0 | 24.0 | > UDP RR latency over IPv4: ns to host | - | - | - | 26 | > |--------|--------|--------|--------| > UDP throughput over IPv6: host to ns | - | - | - | 21.5 | > UDP RR latency over IPv6: host to ns | - | - | - | 29 | > |--------|--------|--------|--------| > UDP throughput over IPv4: host to ns | - | - | - | 28.7 | > UDP RR latency over IPv4: host to ns | - | - | - | 29 | > '--------'--------'--------'--------' > > ...except for the cases with low MTUs, throughput is significantly > higher if we read and send one message at a time on the "spliced" path. > > Next, I would like to: > > - bisect between 32 and 1 for UDP_MAX_FRAMES: maybe 32 affects data > locality too much, but some lower value would still be beneficial by > lowering syscall overhead Ok. > - try with sendmsg() instead of sendmmsg(), at this point. Looking at > the kernel, that doesn't seem to make a real difference. Which sendmmsg() specifically are you looking at changing? > About this series: should we just go ahead and apply it with > UDP_MAX_FRAMES set to 1 for the moment being? It's anyway better than > the existing situation. I think that's a good idea - or rather, not setting UDP_MAX_FRAMES to 1, but clamping the batch size to 1 for pasta - I'm pretty sure we still want the batching for passt. We lose a little bit on small-packet spliced, but we gain on both tap and large-packet spliced. This will unblock the dual stack udp stuff and we can further tune it later. -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson