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From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
To: Alexander van Heukelum <heukelum@mailshack.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
	"H. Peter Anvin" <hpa@zytor.com>,
	LKML <linux-kernel@vger.kernel.org>,
	heukelum@fastmail.fm
Subject: Re: [PATCH] x86: Change x86 to use generic find_next_bit
Date: Thu, 13 Mar 2008 18:14:24 +0530	[thread overview]
Message-ID: <20080313124424.GA18774@skywalker> (raw)
In-Reply-To: <20080309200103.GA895@mailshack.com>

On Sun, Mar 09, 2008 at 09:01:04PM +0100, Alexander van Heukelum wrote:
> x86: Change x86 to use the generic find_next_bit implementation
> 
> The versions with inline assembly are in fact slower on the machines I
> tested them on (in userspace) (Athlon XP 2800+, p4-like Xeon 2.8GHz, AMD
> Opteron 270). The i386-version needed a fix similar to 06024f21 to avoid
> crashing the benchmark.
> 
> Benchmark using: gcc -fomit-frame-pointer -Os. For each bitmap size 
> 1...512, for each possible bitmap with one bit set, for each possible 
> offset: find the position of the first bit starting at offset. If you 
> follow ;). Times include setup of the bitmap and checking of the 
> results.
> 
> 		Athlon		Xeon		Opteron 32/64bit
> x86-specific:	0m3.692s	0m2.820s	0m3.196s / 0m2.480s
> generic:	0m2.622s	0m1.662s	0m2.100s / 0m1.572s
> 
> If the bitmap size is not a multiple of BITS_PER_LONG, and no set 
> (cleared) bit is found, find_next_bit (find_next_zero_bit) returns a 
> value outside of the range [0,size]. The generic version always returns 
> exactly size. The generic version also uses unsigned long everywhere, 
> while the x86 versions use a mishmash of int, unsigned (int), long and 
> unsigned long.
> 

This problem is observed on x86_64 and powerpc also. We need a long
aligned address for test_bit, set_bit find_bit etc.  In ext4 we have
to make sure we align the address passed to

ext4_test_bit
ext4_set_bit
ext4_find_next_zero_bit
ext4_find_next_bit

fs/ext4/mballoc.c have some examples.

-aneesh

  parent reply	other threads:[~2008-03-13 12:44 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-03-09 20:01 Alexander van Heukelum
2008-03-09 20:10 ` Ingo Molnar
2008-03-09 21:03   ` Andi Kleen
2008-03-09 21:32     ` Andi Kleen
2008-03-09 21:13   ` Alexander van Heukelum
2008-03-10  6:29     ` Ingo Molnar
2008-03-09 20:11 ` Ingo Molnar
2008-03-09 20:31   ` Alexander van Heukelum
2008-03-09 20:51     ` Ingo Molnar
2008-03-09 21:29       ` Andi Kleen
2008-03-10 23:17       ` [RFC/PATCH] x86: Optimize find_next_(zero_)bit for small constant-size bitmaps Alexander van Heukelum
2008-03-11  9:56         ` Ingo Molnar
2008-03-11 15:17           ` [PATCH] " Alexander van Heukelum
2008-03-11 15:22             ` [RFC] non-x86: " Alexander van Heukelum
2008-03-11 15:23             ` [PATCH] x86: " Ingo Molnar
2008-03-09 20:28 ` [PATCH] x86: Change x86 to use generic find_next_bit Andi Kleen
2008-03-09 21:31 ` Andi Kleen
2008-03-13 12:44 ` Aneesh Kumar K.V [this message]
2008-03-13 14:27   ` Alexander van Heukelum

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