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

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Date:	Thu, 26 Feb 2015 14:18:20 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Eric Dumazet' <eric.dumazet@...il.com>,
	Daniel Borkmann <daniel@...earbox.net>
CC:	"davem@...emloft.net" <davem@...emloft.net>,
	"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

> 1) Automatic shrinking is a non issue. This will free very little
> memory, compared to previous peak usage (including objects put in
> rhashtable). If hash grown to a certain point, it's likely it will grow
> again later.

Shrink could be done 'on demand' from the 'user' when the number of
elements is known to have significantly reduced.

> 2) Growing can be triggered when any bucket has more than X elems, and
> that is given for free at insert time.
> X could be log2(buckets)/2 I guess. (aka shift/2)

If growing is triggered when a bucket has more than X elems, then setting
X very high will stop automatic growing.
Growing would then only be done in response to user requests.

Whether X should be constant, or log2(buckets)/n is another matter.
Or something like N1 + (N2 * log2(buckets))/64 in any case it
only needs recalculating after an auto-grow.

	David