Re: [PATCH v2 net-next 0/3] ipv4: Hash-based multipath routing

On Sat, Aug 29, 2015 at 1:46 PM, David Miller [off-list ref] wrote:

From: Peter Nørlund <pch-chEQUL3jiZBWk0Htik3J/w@public.gmane.org>
Date: Sat, 29 Aug 2015 22:31:15 +0200

quoted

On Sat, 29 Aug 2015 13:14:29 -0700 (PDT)
David Miller [off-list ref] wrote:

quoted

From: pch-chEQUL3jiZBWk0Htik3J/w@public.gmane.org
Date: Fri, 28 Aug 2015 22:00:47 +0200

quoted

When the routing cache was removed in 3.6, the IPv4 multipath
algorithm changed from more or less being destination-based into
being quasi-random per-packet scheduling. This increases the risk
of out-of-order packets and makes it impossible to use multipath
together with anycast services.

Don't even try to be fancy.

Simply kill the round-robin stuff off completely, and make hash based
routing the one and only mode, no special configuration stuff
necessary.

I like the sound of that! Just to be clear - are you telling me to
stick with L3 and skip the L4 part?

For now it seems best to just do L3 and make ipv4 and ipv6 behave the
same.

This might be simpler if we just go directly to L4 which should be
better load balancing and what most switches are doing anyway. The
hash comes from:

1) If a lookup includes an skb, we just need to call skb_get_hash.
2) If we have a socket and sk->sk_txhash is nonzero then use that.
3) Else compute a hash frome flowi. We don't have the exact functions
for this, but they can be easily derived from __skb_get_hash_flowi4
and __skb_get_hash_flowi6 (i.e. create general get_hash_flowi4 and
get_hash_flowi6 and then call these from skb functions and multipath
lookup).

Tom

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