linux-kernel - Re: lmbench lat_mmap slowdown with CONFIG

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Message-ID: <49790A6F.1080000@goop.org>
Date:	Thu, 22 Jan 2009 16:08:15 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Zachary Amsden <zach@...are.com>, Nick Piggin <npiggin@...e.de>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"jeremy@...source.com" <jeremy@...source.com>,
	"chrisw@...s-sol.org" <chrisw@...s-sol.org>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

H. Peter Anvin wrote:
> Right now a number of the call sites contain a huge push/pop sequence 
> followed by an indirect call.  We can patch in the native code to 
> avoid the branch overhead, but the register constraints and icache 
> footprint is unchanged.

That's true for the pvops hooks emitted in the .S files, but not so true 
for ones in C code (well, there are no explicit push/pops, but the 
presence of the call may cause the compiler to generate them).

The .S hooks can definitely be cleaned up, but I don't think that's 
germane to Nick's observations that the mm code is showing slowdowns.

    J
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