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

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Message-ID: <20081121091851.GA26396@elte.hu>
Date:	Fri, 21 Nov 2008 10:18:51 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [Bug #11308] tbench regression on each kernel release from
	2.6.22 -&gt; 2.6.28


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

> Ingo Molnar a écrit :
>> * Christoph Lameter <cl@...ux-foundation.org> wrote:
>>
>>> hmmm... Well we are almost there.
>>>
>>> 2.6.22:
>>>
>>> Throughput 2526.15 MB/sec 8 procs
>>>
>>> 2.6.28-rc5:
>>>
>>> Throughput 2486.2 MB/sec 8 procs
>>>
>>> 8p Dell 1950 and the number of processors specified on the tbench  
>>> command line.
>>
>> And with net-next we might even be able to get past that magic limit?  
>> net-next is linus-latest plus the latest and greatest networking bits:
>>
>>  $ cat .git/config
>>
>>  [remote "net-next"]
>> 	url = git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next-2.6.git
>> 	fetch = +refs/heads/*:refs/remotes/net-next/*
>>
>> ... so might be worth a test. Just to satisfy our curiosity and to 
>> possibly close the entry :-)
>>
>
> Well, bits in net-next are new stuff for 2.6.29, not really 
> regression fixes, but yes, they should give nice tbench speedups.

yeah, i know - technically these are lots-of-kernel-releases effects 
so not bona fide latest-cycle regressions anyway. But it doesnt matter 
how we call them, we want improvement in these metrics.

> Now, I wish sockets and pipes not going through dcache, not tbench 
> affair of course but real workloads...
>
> running 8 processes on a 8 way machine doing a 
>
> for (;;)
> 	close(socket(AF_INET, SOCK_STREAM, 0));
>
> is slow as hell, we hit so many contended cache lines ...
>
> ticket spin locks are slower in this case (dcache_lock for example 
> is taken twice when we allocate a socket(), once in d_alloc(), 
> another one in d_instantiate())

hm, weird - since there's no real VFS namespace impact i fail to 
realize the fundamental need that causes us to hit the dcache_lock. 
(perhaps there's none and this is fixable)

The general concept of mapping sockets to fds is a fundamental and 
powerful abstraction. There are APIs that also connect them to the VFS 
namespace (such as unix domain sockets) - but those should be special 
cases, not impacting normal TCP sockets.

	Ingo
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