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

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Message-ID: <1424886096.5565.104.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 25 Feb 2015 09:41:36 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Daniel Borkmann <daniel@...earbox.net>
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 Wed, 2015-02-25 at 17:14 +0100, Daniel Borkmann wrote:

> The nelems can be a percpu_counter where we have a batched sync
> point and can make that dependent on the current table shift as
> we don't need to be overly precise, we can just read the sync'ed
> value. Currently, nelems are also being used by rhashtable users
> outside of the core code to track if we are still allowed to
> insert new elements, but I think we might also want to address
> that at some point.

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.

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)

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

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