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:

quoted

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'.