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From: Alex Rosenbaum <rosenbaumalex@gmail.com>
To: Tom Talpey <tom@talpey.com>
Cc: Mark Zhang <markz@mellanox.com>, Jason Gunthorpe <jgg@ziepe.ca>,
	RDMA mailing list <linux-rdma@vger.kernel.org>,
	Eran Ben Elisha <eranbe@mellanox.com>,
	Yishai Hadas <yishaih@mellanox.com>,
	Alex Rosenbaum <alexr@mellanox.com>,
	Maor Gottlieb <maorg@mellanox.com>,
	Leon Romanovsky <leonro@mellanox.com>
Subject: Re: [RFC v2] RoCE v2.0 Entropy - IPv6 Flow Label and UDP Source Port
Date: Tue, 25 Feb 2020 15:20:53 +0200
Message-ID: <CAFgAxU_fZUEYrMVpoStQJGD5S5dqrkXOuzWrk0MLHF8niW7ikw@mail.gmail.com> (raw)
In-Reply-To: <33f075e2-b5c0-53cd-6954-7ac57eeb008f@talpey.com>

Mark and I where playing with your test, and plotting the results
I'm sharing the png's on a temp github here:
https://github.com/rosenbaumalex/hashtest/
[I wasn't sure of a better place to share them]

The README.md explains the port range we used, the 3 hash's used, and
a line about the results.
In general, the higher the 'noise' the worse the distribution is.
It seems like Mark's hash suggestion (src*31 + dst) works best. then
the folding one, and last the non-folding one.

I am trying to cache a few switch related hash experts to get
additional feedback.

Alex

On Fri, Feb 21, 2020 at 4:47 PM Tom Talpey <tom@talpey.com> wrote:
>
> On 2/19/2020 8:04 PM, Mark Zhang wrote:
> > On 2/20/2020 1:41 AM, Tom Talpey wrote:
> >> On 2/19/2020 8:06 AM, Jason Gunthorpe wrote:
> >>> On Wed, Feb 19, 2020 at 02:06:28AM +0000, Mark Zhang wrote:
> >>>> The symmetry is important when calculate flow_label with DstQPn/SrcQPn
> >>>> for non-RDMA CM Service ID (check the first mail), so that the server
> >>>> and client will have same flow_label and udp_sport. But looks like it is
> >>>> not important in this case.
> >>>
> >>> If the application needs a certain flow label it should not rely on
> >>> auto-generation, IMHO.
> >>>
> >>> I expect most networks will not be reversible anyhow, even with the
> >>> same flow label?
> >>
> >> These are network flow labels, not under application control. If they
> >> are under application control, that's a security issue.
> >>
> >
> > As Jason said application is able to control it in ipv6. Besides
> > application is also able to control it for non-RDMA CM Service ID in ipv4.
>
> Ok, well I guess that's a separate issue, let's not rathole on
> it here then.
>
> > Hi Jason, same flow label get same UDP source port, with same UDP source
> > port (along with same sIP/dIP/sPort), are networks reversible?
> >
> >> But I agree, if the symmetric behavior is not needed, it should be
> >> ignored and a better (more uniformly distributed) hash should be chosen.
> >>
> >> I definitely like the simplicity and perfect flatness of the newly
> >> proposed (src * 31) + dst. But that "31" causes overflow into bit 21,
> >> doesn't it? (31 * 0xffff == 0x1f0000) >
> >
> > I think overflow doesn't matter? We have overflow anyway if
> > multiplicative is used.
>
> Hmm, it does seem to matter because dropping bits tilts the
> distribution curve. Plugging ((src * 31) + dst) & 0xFFFFF into
> my little test shows some odd behaviors. It starts out flat,
> then the collisions start to rise around 49000, leveling out
> at 65000 to a value roughly double the initial one (528 -> 1056).
> It sits there until 525700, where it falls back to the start
> value (528). I don't think this is optimal :-)
>
> Tom.

      reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-08 14:26 Alex Rosenbaum
2020-01-15  9:48 ` Mark Zhang
2020-02-06 14:18 ` Tom Talpey
2020-02-06 14:35   ` Jason Gunthorpe
2020-02-06 14:39   ` Alex Rosenbaum
2020-02-06 15:19     ` Tom Talpey
2020-02-08  9:58       ` Alex Rosenbaum
2020-02-12 15:47         ` Tom Talpey
2020-02-13 11:03           ` Alex Rosenbaum
2020-02-13 15:26             ` Tom Talpey
2020-02-13 15:41               ` Jason Gunthorpe
2020-02-14 14:23                 ` Mark Zhang
2020-02-15  6:27                   ` Mark Zhang
2020-02-18 14:16                     ` Tom Talpey
2020-02-18 17:41                       ` Tom Talpey
2020-02-19  1:51                         ` Mark Zhang
2020-02-19  2:01                           ` Tom Talpey
2020-02-19  2:06                             ` Mark Zhang
2020-02-19 13:06                               ` Jason Gunthorpe
2020-02-19 17:41                                 ` Tom Talpey
2020-02-19 17:55                                   ` Jason Gunthorpe
2020-02-20  1:04                                   ` Mark Zhang
2020-02-21 14:47                                     ` Tom Talpey
2020-02-25 13:20                                       ` Alex Rosenbaum [this message]

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