linux-kernel - Re: [PATCH v7 10/16] dlm: use new hashtable implementation

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Date:	Mon, 29 Oct 2012 11:53:54 -0400
From:	Sasha Levin <levinsasha928@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	torvalds@...ux-foundation.org, tj@...nel.org,
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Subject: Re: [PATCH v7 10/16] dlm: use new hashtable implementation

On Mon, Oct 29, 2012 at 9:07 AM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> * Mathieu Desnoyers (mathieu.desnoyers@...icios.com) wrote:
>> * Sasha Levin (levinsasha928@...il.com) wrote:
>> [...]
>> > @@ -158,34 +159,21 @@ static int dlm_allow_conn;
>> >  static struct workqueue_struct *recv_workqueue;
>> >  static struct workqueue_struct *send_workqueue;
>> >
>> > -static struct hlist_head connection_hash[CONN_HASH_SIZE];
>> > +static struct hlist_head connection_hash[CONN_HASH_BITS];
>> >  static DEFINE_MUTEX(connections_lock);
>> >  static struct kmem_cache *con_cache;
>> >
>> >  static void process_recv_sockets(struct work_struct *work);
>> >  static void process_send_sockets(struct work_struct *work);
>> >
>> > -
>> > -/* This is deliberately very simple because most clusters have simple
>> > -   sequential nodeids, so we should be able to go straight to a connection
>> > -   struct in the array */
>> > -static inline int nodeid_hash(int nodeid)
>> > -{
>> > -   return nodeid & (CONN_HASH_SIZE-1);
>> > -}
>>
>> There is one thing I dislike about this change: you remove a useful
>> comment. It's good to be informed of the reason why a direct mapping
>> "value -> hash" without any dispersion function is preferred here.

Yes, I've removed the comment because it's no longer true with the patch :)

> And now that I come to think of it: you're changing the behavior : you
> will now use a dispersion function on the key, which goes against the
> intent expressed in this comment.

The comment gave us the information that nodeids are mostly
sequential, we no longer need to rely on that.

> It might be good to change hash_add(), hash_add_rcu(),
> hash_for_each_possible*() key parameter for a "hash" parameter, and let
> the caller provide the hash value computed by the function they like as
> parameter, rather than enforcing hash_32/hash_64.

Why? We already proved that hash_32() is more than enough as a hashing
function, why complicate things?

Even doing hash_32() on top of another hash is probably a good idea to
keep things simple.

Thanks,
Sasha
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