Re: [dm-devel] [PATCH v7 15/16] openvswitch: use new hashtable implementation

[Sasha Levin <levinsasha928 gmail com>]

On Mon, Oct 29, 2012 at 11:59 AM, Mathieu Desnoyers
<mathieu desnoyers efficios com> wrote:
> * Sasha Levin (levinsasha928 gmail com) wrote:
>> Hi Mathieu,
>>
>> On Mon, Oct 29, 2012 at 9:29 AM, Mathieu Desnoyers
>> <mathieu desnoyers efficios com> wrote:
>> > * Sasha Levin (levinsasha928 gmail com) wrote:
>> > [...]
>> >> -static struct hlist_head *hash_bucket(struct net *net, const char *name)
>> >> -{
>> >> -     unsigned int hash = jhash(name, strlen(name), (unsigned long) net);
>> >> -     return &dev_table[hash & (VPORT_HASH_BUCKETS - 1)];
>> >> -}
>> >> -
>> >>  /**
>> >>   *   ovs_vport_locate - find a port that has already been created
>> >>   *
>> >> @@ -84,13 +76,12 @@ static struct hlist_head *hash_bucket(struct net *net, const char *name)
>> >>   */
>> >>  struct vport *ovs_vport_locate(struct net *net, const char *name)
>> >>  {
>> >> -     struct hlist_head *bucket = hash_bucket(net, name);
>> >>       struct vport *vport;
>> >>       struct hlist_node *node;
>> >> +     int key = full_name_hash(name, strlen(name));
>> >>
>> >> -     hlist_for_each_entry_rcu(vport, node, bucket, hash_node)
>> >> -             if (!strcmp(name, vport->ops->get_name(vport)) &&
>> >> -                 net_eq(ovs_dp_get_net(vport->dp), net))
>> >> +     hash_for_each_possible_rcu(dev_table, vport, node, hash_node, key)
>> >
>> > Is applying hash_32() on top of full_name_hash() needed and expected ?
>>
>> Since this was pointed out in several of the patches, I'll answer it
>> just once here.
>>
>> I've intentionally "allowed" double hashing with hash_32 to keep the
>> code simple.
>>
>> hash_32() is pretty simple and gcc optimizes it to be almost nothing,
>> so doing that costs us a multiplication and a shift. On the other
>> hand, we benefit from keeping our code simple - how would we avoid
>> doing this double hash? adding a different hashtable function for
>> strings? or a new function for already hashed keys? I think we benefit
>> a lot from having to mul/shr instead of adding extra lines of code
>> here.
>
> This could be done, as I pointed out in another email within this
> thread, by changing the "key" argument from add/for_each_possible to an
> expected "hash" value, and let the caller invoke hash_32() if they want.
> I doubt this would add a significant amount of complexity for users of
> this API, but would allow much more flexibility to choose hash
> functions.

Most callers do need to do the hashing though, so why add an
additional step for all callers instead of doing another hash_32 for
the ones that don't really need it?

Another question is why do you need flexibility? I think that
simplicity wins over flexibility here.

Thanks,
Sasha