On Thu, Jan 14, 2016 at 11:15 AM, Haiyang Zhang <haiyangz@xxxxxxxxxxxxx> wrote: > > >> -----Original Message----- >> From: Tom Herbert [mailto:tom@xxxxxxxxxxxxxxx] >> Sent: Thursday, January 14, 2016 1:49 PM >> To: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx> >> Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>; One Thousand Gnomes >> <gnomes@xxxxxxxxxxxxxxxxxxx>; David Miller <davem@xxxxxxxxxxxxx>; >> vkuznets@xxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; KY Srinivasan >> <kys@xxxxxxxxxxxxx>; devel@xxxxxxxxxxxxxxxxxxxxxx; linux- >> kernel@xxxxxxxxxxxxxxx >> Subject: Re: [PATCH net-next] hv_netvsc: don't make assumptions on >> struct flow_keys layout >> >> On Thu, Jan 14, 2016 at 10:35 AM, Haiyang Zhang <haiyangz@xxxxxxxxxxxxx> >> wrote: >> > >> > >> >> -----Original Message----- >> >> From: Eric Dumazet [mailto:eric.dumazet@xxxxxxxxx] >> >> Sent: Thursday, January 14, 2016 1:24 PM >> >> To: One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx> >> >> Cc: Tom Herbert <tom@xxxxxxxxxxxxxxx>; Haiyang Zhang >> >> <haiyangz@xxxxxxxxxxxxx>; David Miller <davem@xxxxxxxxxxxxx>; >> >> vkuznets@xxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; KY Srinivasan >> >> <kys@xxxxxxxxxxxxx>; devel@xxxxxxxxxxxxxxxxxxxxxx; linux- >> >> kernel@xxxxxxxxxxxxxxx >> >> Subject: Re: [PATCH net-next] hv_netvsc: don't make assumptions on >> >> struct flow_keys layout >> >> >> >> On Thu, 2016-01-14 at 17:53 +0000, One Thousand Gnomes wrote: >> >> > > These results for Toeplitz are not plausible. Given random input >> you >> >> > > cannot expect any hash function to produce such uniform results. >> I >> >> > > suspect either your input data is biased or how your applying the >> >> hash >> >> > > is. >> >> > > >> >> > > When I run 64 random IPv4 3-tuples through Toeplitz and Jenkins I >> >> get >> >> > > something more reasonable: >> >> > >> >> > IPv4 address patterns are not random. Nothing like it. A long long >> >> time >> >> > ago we did do a bunch of tuning for network hashes using big porn >> site >> >> > data sets. Random it was not. >> >> > >> >> >> >> I ran my tests with non random IPV4 addresses, as I had 2 hosts, >> >> one server, one client. (typical benchmark stuff) >> >> >> >> The only 'random' part was the ports, so maybe ~20 bits of entropy, >> >> considering how we allocate ports during connect() to a given >> >> destination to avoid port reuse. >> >> >> >> > It's probably hard to repeat that exercise now with geo specific >> >> routing, >> >> > and all the front end caches and redirectors on big sites but I'd >> >> > strongly suggest random input is not a good test, and also that you >> >> need >> >> > to worry more about hash attacks than perfect distributions. >> >> >> >> Anyway, the exercise is not to find a hash that exactly splits 128 >> flows >> >> into 16 buckets, according to the number of flows per bucket. >> >> >> >> Maybe only 4 flows are sending at 3Gbits, and others are sending at >> 100 >> >> kbits. There is no way the driver can predict the future. >> >> >> >> This is why we prefer to select a queue given the cpu sending the >> >> packet. This permits a natural shift based on actual load, and is the >> >> default on linux (see XPS in Documentation/networking/scaling.txt) >> >> >> >> Only this driver has a selection based on a flow 'hash'. >> > >> > Also, the port number selection may not be random either. For example, >> > the well-known network throughput test tool, iperf, use port numbers >> with >> > equal increment among them. We tested these non-random cases, and >> found >> > the Toeplitz hash has distributed evenly, but Jenkins hash has non- >> even >> > distribution. >> > >> > I'm aware of the test from Tom Herbert <tom@xxxxxxxxxxxxxxx>, which >> > showing similar results of Toeplitz v.s. Jenkins with random inputs. >> > >> > In summary, the Toeplitz performs better in case of non-random inputs, >> > and performs similar to Jenkins in random inputs (which may not be the >> > case in real world). So we still prefer to use Toeplitz hash. >> > >> You are basing your conclusions on one toy benchmark. I don't believe >> that an realistically loaded web server is going to consistently give >> you tuples that happen to somehow fit into a nice model so that the >> bias benefits your load distribution. >> >> > To minimize the computational overhead, we may consider put the hash >> > in a per-connection cache in TCP layer, so it only needs one time >> > computation. But, even with the computation overhead at this moment, >> > the throughput based on Toeplitz hash is better than Jenkins: >> > Throughput (Gbps) comparison: >> > #conn Toeplitz Jenkins >> > 32 26.6 23.2 >> > 64 32.1 23.4 >> > 128 29.1 24.1 >> > >> You don't need to do that. We already store a random hash value in the >> connection context. If you want to make it non-random then just >> replace that with a simple global counter. This will have the exact >> same effect that you see in your tests without needing any expensive >> computation. > > Could you point me to the data field of connection context where this > hash value is stored? Is it computed only one time? > sk_txhash in struct sock. It is set to a random number on TCP or UDP connect call, It can be reset to a different random value when connection is seen to be have trouble (sk_rethink_txhash). Also when you say "Toeplitz performs better in case of non-random inputs" please quantify exactly how your input data is not random. What header changes with each connection in your test... > Thanks! > > - Haiyang > > > _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
