On Thu, Jan 14, 2016 at 12:23 PM, Haiyang Zhang <haiyangz@xxxxxxxxxxxxx> wrote: > > >> -----Original Message----- >> From: Tom Herbert [mailto:tom@xxxxxxxxxxxxxxx] >> Sent: Thursday, January 14, 2016 2:41 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 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... > > Thank you for the info! > > For non-random inputs, I used the port selection of iperf that increases > the port number by 2 for each connection. Only send-port numbers are > different, other values are the same. I also tested some other fixed > increment, Toeplitz spreads the connections evenly. For real applications, > if the load came from local area, then the IP/port combinations are > likely to have some non-random patterns. > Okay, by only changing source port I can produce the same uniformity: 64 connections with a step of 2 for changing source port gives: Buckets: 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 _but_, I can also find also make steps that severely mess up load distribution. Step 1024 gives: Buckets: 0 8 8 0 0 8 8 0 8 0 0 8 8 0 0 8 The fact that we can negatively affect the output of Toeplitz so predictably is actually a liability and not a benefit. This sort of thing can be the basis of a DOS attack and is why we kicked out XOR hash in favor of Jenkins. > For our driver, we are thinking to put the Toeplitz hash to the sk_txhash, > so it only needs to be computed only once, or during sk_rethink_txhash. > So, the computational overhead happens almost only once. > > Thanks, > - Haiyang > > _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
