[ovs-dev] [PATCH V3 3/5] datapath: Allow each vport to have an array of 'port_id's.

[Thomas Graf]

On 03/13/2014 10:04 PM, Alex Wang wrote:
> After asking around, there are two solutions listed as follows:
>
> 1. Ask user to specify the number of handlers to be power of 2.  This
> way, we can use the bitwise AND for modular operation.
>
> 2. Use the reciprocal division.  At time of setting the
> 'upcall_portids', we compute the reciprocal_value of the 'n_ids'.
>       And the ovs_vport_find_portid() is changed to:
>
> """
> @@ -445,13 +437,14 @@ int ovs_vport_get_upcall_portids(const struct
> vport *vport,
>   u32 ovs_vport_find_portid(const struct vport *p, struct sk_buff *skb)
>   {
>          struct vport_portids *ids;
> +       u32 hash;
>          ids = rcu_dereference_ovsl(p->upcall_portids);
>          if (ids->n_ids == 1 && *ids->ids == 0)
>                  return 0;
> -       return ids->ids[skb_get_rxhash(skb) % ids->n_ids];
> +       hash = skb_get_rxhash(skb);
> +       return ids->ids[hash - ids->n_ids * reciprocal_div(hash,
> ids->rn_ids)];
>   }"""
>
> We tend to take the second one, in that, we want to allow user to
> flexibly configure the number of threads in userspace,
> and the change of division to "one subtract, two multiplication and one
> shift" saves the cpu cycles.  (There may be better
> way of using reciprocal_div() than the one I gave)

I agree. If this was the fast path I would have suggested to go with
(1) but given we only hit this frequently in exception cases the
added flexibility in defining an arbitrary number of threads seems
worth it.