linux-kernel - Re: [Bug #11308] tbench regression on each kernel release from 2.6.22 -> 2.6.28

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Message-ID: <20081117161135.GE12081@elte.hu>
Date:	Mon, 17 Nov 2008 17:11:35 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	David Miller <davem@...emloft.net>, rjw@...k.pl,
	linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org,
	cl@...ux-foundation.org, efault@....de, a.p.zijlstra@...llo.nl,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [Bug #11308] tbench regression on each kernel release from
	2.6.22 -&gt; 2.6.28


* Eric Dumazet <dada1@...mosbay.com> wrote:

>> It all looks like pure old-fashioned straight overhead in the 
>> networking layer to me. Do we still touch the same global cacheline 
>> for every localhost packet we process? Anything like that would 
>> show up big time.
>
> Yes we do, I find strange we dont see dst_release() in your NMI 
> profile
>
> I posted a patch ( commit 5635c10d976716ef47ae441998aeae144c7e7387 
> net: make sure struct dst_entry refcount is aligned on 64 bytes) (in 
> net-next-2.6 tree) to properly align struct dst_entry refcounter and 
> got 4% speedup on tbench on my machine.

Ouch, +4% from a oneliner networking change? That's a _huge_ speedup 
compared to the things we were after in scheduler land. A lot of 
scheduler folks worked hard to squeeze the last 1-2% out of the 
scheduler fastpath (which was not trivial at all). The _full_ 
scheduler accounts for only about 7% of the total system overhead here 
on a 16-way box...

So why should we be handling this anything but a plain networking 
performance regression/weakness? The localhost scalability bottleneck 
has been reported a _long_ time ago.

	Ingo
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