linux-kernel - Re: [patch 0/7] [RFC] SLUB: Improve allocpercpu to reduce per cpu access overhead

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Date:	Mon, 12 Nov 2007 21:14:47 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	"Luck, Tony" <tony.luck@...el.com>
CC:	Christoph Lameter <clameter@....com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	mathieu.desnoyers@...ymtl.ca, penberg@...helsinki.fi
Subject: Re: [patch 0/7] [RFC] SLUB: Improve allocpercpu to reduce per cpu
 access overhead

Luck, Tony a écrit :
>>> Ahh so the need to be able to expand per cpu memory storage on demand 
>>> is not as critical as we thought.
>>>
>> Yes, but still desirable for future optimizations.
>>
>> For example, I do think using a per cpu memory storage on net_device refcnt & 
>> last_rx could give us some speedups.
> 
> We do want to keep a very tight handle on bloat in per-cpu
> allocations.  By definition the total allocation is multiplied
> by the number of cpus.  Only ia64 has outrageous numbers of
> cpus in a single system image today ... but the trend in
> multi-core chips looks to have a Moore's law arc to it, so
> everyone is going to be looking at lots of cpus before long.
> 

I dont think this is a problem. Cpus numbers and ram size are related, even if 
Moore didnt predicted it;

Nobody wants to ship/build a 4096 cpus machine with 256 MB of ram inside.
Or call it a GPU and dont expect it to run linux :)

99,9% of linux machines running on earth have less than 8 cpus and less than 
1000 ethernet/network devices.

In case we increase the number of cpus on a machine, the limiting factor is 
the fact that cpus have to continually exchange on memory bus those highly 
touched cache lines that contain refcounters or stats.

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