netdev - Re: [PATCH] net: make ip_rt_acct a normal percpu var

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Date:	Thu, 20 Nov 2008 00:23:48 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] net: make ip_rt_acct a normal percpu var

Rusty Russell a écrit :
> On Thursday 20 November 2008 08:50:23 David Miller wrote:
>> Do you really need this to forward some work you are doing?  If not
>> can we just let sleeping dogs lie on this one? :)
> 
> Yes, I have patches to convert the dynamic percpu data to use the same 
> mechanism as static percpu data.  Unfortunately we don't have a mechanism for 
> enlarging the percpu region (which is why this wasn't done earlier), so we use 
> a heuristic to figure out how much extra percpu region to allocate at boot.
> 
> And 4k makes this one of the Big Pigs in dynamic per-cpu allocations.
> 
> (SNMP mibs are even worse, but that's a separate debate...)
> 
> I can try to implement a bss-like DEFINE_PER_CPU_ZERO(), but it seems silly to 
> talk about tight boot loader size restrictions for SMP kernels.
> 

Then, if we really want to run 4096 cpus on a machine, we dont want to allocate
16 MBytes of memory for these ip_rt_acct counters, or even more for SNMP mibs.

Maybe its time to design a new mechanism, to avoid the basic "one variable" shared
by all cpus, and avoid the overkill "one separate variable for each cpu", and loop
4096 times to do the sum of this variable...

Something that would allocate a maximum of eight blocs.

Then atomic ops would be necessary for updates of SNMP counters (only if NR_CPUS > 8)


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