netdev - RE: [PATCH net 2/2] rhashtable: remove indirection for grow/shrink decision functions

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Date:	Wed, 25 Feb 2015 15:41:28 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Daniel Borkmann' <daniel@...earbox.net>,
	"davem@...emloft.net" <davem@...emloft.net>
CC:	"tgraf@...g.ch" <tgraf@...g.ch>,
	"pablo@...filter.org" <pablo@...filter.org>,
	"johunt@...mai.com" <johunt@...mai.com>,
	"kaber@...sh.net" <kaber@...sh.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH net 2/2] rhashtable: remove indirection for grow/shrink
 decision functions

From: Daniel Borkmann
> Currently, all real users of rhashtable default their grow and shrink
> decision functions to rht_grow_above_75() and rht_shrink_below_30(),
> so that there's currently no need to have this explicitly selectable.
> 
> It can/should be generic and private inside rhashtable until a real
> use case pops up. Since we can make this private, we'll save us this
> additional indirection layer and can improve insertion/deletion time
> as well.
...
> +static bool rht_grow_above_75(const struct rhashtable *ht, size_t new_size)
>  {
>  	/* Expand table when exceeding 75% load */
>  	return atomic_read(&ht->nelems) > (new_size / 4 * 3) &&
>  	       (!ht->p.max_shift || atomic_read(&ht->shift) < ht->p.max_shift);
>  }

Why not cache the 'number of items before we need to expand' value
after each expansion, setting it to 'infinite' when expansion is disabled.
Then the above check is a simple comparison.
You probably don't even need an atomic_read() - provided something is
double checked once the first test determines that an expansion is needed.

	David

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