netdev - Re: htb parallelism on multi-core platforms

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Message-ID: <Pine.LNX.4.64.0904301333150.26466@tyr.diku.dk>
Date:	Thu, 30 Apr 2009 13:44:53 +0200 (CEST)
From:	Jesper Dangaard Brouer <hawk@...u.dk>
To:	Radu Rendec <radu.rendec@...s.ro>
Cc:	Calin Velea <vcalinus@...enii.ro>,
	Jarek Poplawski <jarkao2@...il.com>,
	Denys Fedoryschenko <denys@...p.net.lb>,
	netdev <netdev@...r.kernel.org>
Subject: Re: htb parallelism on multi-core platforms

On Thu, 30 Apr 2009, Radu Rendec wrote:

> On Thu, 2009-04-30 at 01:49 +0300, Calin Velea wrote:
>>    I tested with e1000 only, on a single quad-core CPU - the L2 cache was
>> shared between the cores.
>>
>>   For 8 cores I suppose you have 2 quad-core CPUs. If the cores actually
>> used belong to different physical CPUs, L2 cache sharing does not occur -
>> maybe this could explain the performance drop in your case.
>>   Or there may be other explanation...
>
> It is correct, I have 2 quad-core CPUs. If adjacent kernel-identified
> CPUs are on the same physical CPU (e.g. CPU0, CPU1, CPU2 and CPU3) - and
> it is very probable - then I think the L2 cache was actually shared.
> That's because the used CPUs where either 0-3 or 4-7 but never a mix of
> them. So perhaps there is another explanation (maybe driver/hardware).

WRONG assumption regarding CPU id's

Look in /proc/cpuinfo for the correct answer.

(From a:
  model name      : Intel(R) Xeon(R) CPU           E5420  @ 2.50GHz)

cat /proc/cpuinfo | egrep -e '(processor|physical id|core id)'
processor       : 0
physical id     : 0
core id         : 0

processor       : 1
physical id     : 1
core id         : 0

processor       : 2
physical id     : 0
core id         : 2

processor       : 3
physical id     : 1
core id         : 2

processor       : 4
physical id     : 0
core id         : 1

processor       : 5
physical id     : 1
core id         : 1

processor       : 6
physical id     : 0
core id         : 3

processor       : 7
physical id     : 1
core id         : 3

E.g. Here CPU0 and CPU4 is sharing the same L2 cache.


Hilsen
   Jesper Brouer

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