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

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Date:	Wed, 25 Feb 2015 19:56:08 +0100
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Laight <David.Laight@...LAB.COM>,
	"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

On 02/25/2015 06:41 PM, Eric Dumazet wrote:
...
> There is no need to track number of elements, using either an atomic_t
> or percpu_counter. This adds unnecessary burden.
>
> 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.

So you are saying that shrinking is most likely a rather undesirable
use-case in rhashtable?

> 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)

Yes, I think such a facility can be added easily.

> A global limit on number of elements should be controlled by rhashtable
> users - if needed -, not in the rhashtable itself.

Makes sense, thanks for the suggestion! I'll experiment with the above
per bucket tracking a bit if you're okay with that, and also unfiddle
the rht counter internals from netlink.

Thanks,
Daniel
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