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

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Message-ID: <20150311064241.GA7153@gondor.apana.org.au>
Date:	Wed, 11 Mar 2015 17:42:41 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	daniel@...earbox.net, David.Laight@...lab.com, davem@...emloft.net,
	tgraf@...g.ch, pablo@...filter.org, johunt@...mai.com,
	kaber@...sh.net, netdev@...r.kernel.org
Subject: Re: [PATCH net 2/2] rhashtable: remove indirection for grow/shrink
 decision functions

Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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.

I agree in the strongest terms :)

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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