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

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Date:	Mon, 29 Oct 2012 09:07:36 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	torvalds@...ux-foundation.org, tj@...nel.org,
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Subject: Re: [PATCH v7 10/16] dlm: use new hashtable implementation

* 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.

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.

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.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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