Flow ID, LACP, and igb

[T.C. Gubatayao]

On Aug 29, 2013, at 4:21 PM, Alan Somers <asomers at freebsd.org> wrote:

> They're faster, but even with this change, jenkins_hash is still 6 times
> slower than FNV hash.

Actually, I think your test isn't accurately simulating memory access, which
might be skewing the results.

For example, from net/if_lagg.c:

                p = hash32_buf(&eh->ether_shost, ETHER_ADDR_LEN, p);
                p = hash32_buf(&eh->ether_dhost, ETHER_ADDR_LEN, p);

These two calls can't both be aligned, since ETHER_ADDR_LEN is 6 octets.  The
same is true for the other hashed fields in the IP and TCP/UDP headers.
Assuming the mbuf data pointer is aligned, the IP addresses and ports are both
on 2-byte alignments (without VLAN or IP options).  In your test, they're all  
aligned and in the same cache line.

When I modify the test to simulate an mbuf, lookup3 beats FNV and hash32, and
SipHash is only 2-3 times slower.

> Also, your technique of copying the hashable fields into a separate buffer
> would need modification to work with different types of packet and different
> LAGG_F_HASH[234] flags.  Because different packets have different hashable
> fields, struct key would need to be expanded to include the vlan tag, IPV6
> addresses, and IPv6 flowid.  lagg_hashmbuf would then have to zero the unused
> fields.

Agreed, but this is relatively simple with a buffer on the stack, and does not
require zeroes or padding.  See my modified test, attached.

T.C.