linux-kernel - RE: 2.6.25.4-rt2

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Date:	Tue, 20 May 2008 08:44:42 +0800
From:	"MA QING A" <Qing.a.Ma@...atel-sbell.com.cn>
To:	"Steven Rostedt" <rostedt@...dmis.org>,
	"Kasper Sandberg" <lkml@...anurb.dk>
Cc:	"LKML" <linux-kernel@...r.kernel.org>,
	"RT" <linux-rt-users@...r.kernel.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>
Subject: RE: 2.6.25.4-rt2

Forgive me to ask a stupid question.
Why using the 24 mainline kernel, the time is less than using the rt-patched kernel?


-----Original Message-----
From: linux-kernel-owner@...r.kernel.org [mailto:linux-kernel-owner@...r.kernel.org] On Behalf Of Steven Rostedt
Sent: 2008年5月20日 7:44
To: Kasper Sandberg
Cc: LKML; RT; Ingo Molnar; Thomas Gleixner
Subject: Re: 2.6.25.4-rt2


On Tue, 20 May 2008, Kasper Sandberg wrote:

> On Mon, 2008-05-19 at 18:43 -0400, Steven Rostedt wrote:
> > We are pleased to announce the 2.6.25.4-rt2 tree, which can be

> Forgive me if i ask an obvious stupid question. But what is the status
> of this -rt tree for .25 in relation with the BKL stuff?

Well, the BKL is still a semaphore in 25.

For my hackbench runs, I have:

[root@...hel51 c]# cat hack-test-2.6.25.4-rt2
Time: 4.937
Time: 4.842
Time: 4.877
Time: 4.905
Time: 4.924
Time: 4.781
Time: 4.927
Time: 4.871
Time: 5.181
Time: 4.866

Which is a bit slower than 2.6.24.7-rt7:

[root@...hel51 c]# cat hack-test-2.6.24.7-rt7
Time: 4.789
Time: 4.824
Time: 4.807
Time: 4.867
Time: 4.802
Time: 4.799
Time: 4.823
Time: 4.855
Time: 4.873
Time: 4.833

But then I checked the base kernels themselves:

[root@...hel51 c]# cat hack-test-2.6.24.7
Time: 3.817
Time: 3.921
Time: 3.887
Time: 3.920
Time: 3.874
Time: 3.858
Time: 3.912
Time: 3.926
Time: 3.888
Time: 3.901

[root@...hel51 c]# cat hack-test-2.6.25.4
Time: 6.225
Time: 6.319
Time: 6.257
Time: 6.534
Time: 6.077
Time: 6.787
Time: 6.927
Time: 6.218
Time: 5.929
Time: 6.554


Where there's a regression somewhere. For 25, the RT patch is actually
*better* than mainline!

So I was thinking it had to do with the BKL regression, and then I ran:

[root@...hel51 c]# cat hack-test-2.6.26-rc3
Time: 6.789
Time: 7.123
Time: 6.197
Time: 5.496
Time: 6.708
Time: 5.609
Time: 6.679
Time: 6.206
Time: 6.351
Time: 5.969


Here it is no better, and the BKL has been converted into a spin lock.

But I'm very much busy working on -rt right now to dig deeper into this
regression.

-- Steve

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