netdev - Re: [PATCH 0/5] Make nicer CONFIG_NET

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Date:	Thu, 01 Nov 2007 07:58:59 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Pavel Emelyanov <xemul@...nvz.org>,
	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>, devel@...nvz.org
Subject: Re: [PATCH 0/5] Make nicer CONFIG_NET_NS=n case code

Eric W. Biederman a écrit :
> Eric Dumazet <dada1@...mosbay.com> writes:
> 
>> Eric W. Biederman a écrit :
>>> Eric Dumazet <dada1@...mosbay.com> writes:
>>>
>>>
>>>> Definitly wanted here. Thank you.
>>>> One more refcounting on each socket creation/deletion was expensive.
>>> Really?  Have you actually measured that?  If the overhead is
>>> measurable and expensive we may want to look at per cpu counters or
>>> something like that.  So far I don't have any numbers that say any
>>> of the network namespace work inherently has any overhead.
>> It seems that on some old opterons (two 246 for example),
>> "if (atomic_dec_and_test(&net->count))" is rather expensive yes :(
> 
> I won't argue that atomic_dec_and_test is costly.  My gut feel is that
> socket creation/destruction is sufficiently rare that such a test
> would be lost in the noise.  Doing anything more sophisticated is
> likely to be less readable, and unless we can measure some overhead
> my preference right now is to keep the code stupid and simple.  Which
> usually has a good icache footprint.

I agree with you that with current state, this atomic_inc/atomic_dec_and_test 
wont come in profiles unless a trivial bench is writen

for(;;){close(socket(....));}

If David or another dev can eliminate the atomic inc/dec on device refcount 
cost for each packet traveling, the socket creation/destruction would 
certainly raise.

Other contention points is the mnt_count (yet another refcount) in "struct 
vfsmount", a truly useless refcount as I never had (and nobody had) to 
un-mount sock_mnt :)


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