linux-kernel - Re: [PATCH v7 15/16] openvswitch: use new hashtable implementation

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Date:	Mon, 29 Oct 2012 11:43:59 -0400
From:	Sasha Levin <levinsasha928@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	torvalds@...ux-foundation.org, tj@...nel.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
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	ebiederm@...ssion.com, aarcange@...hat.com, ericvh@...il.com,
	netdev@...r.kernel.org, josh@...htriplett.org,
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	dev@...nvswitch.org, rds-devel@....oracle.com, lw@...fujitsu.com
Subject: Re: [PATCH v7 15/16] openvswitch: use new hashtable implementation

Hi Mathieu,

On Mon, Oct 29, 2012 at 9:29 AM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> * Sasha Levin (levinsasha928@...il.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.


Thanks,
Sasha
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