From: Anton Altaparmakov <aia21@cam.ac.uk>
To: Grant Grundler <iod00d@hp.com>
Cc: "Chen, Kenneth W" <kenneth.w.chen@intel.com>,
"'Christoph Hellwig'" <hch@infradead.org>,
"'Akinobu Mita'" <mita@miraclelinux.com>,
Linux Kernel Development <linux-kernel@vger.kernel.org>,
linux-ia64@vger.kernel.org
Subject: Re: [PATCH 1/12] generic *_bit()
Date: Wed, 1 Feb 2006 22:49:08 +0000 (GMT) [thread overview]
Message-ID: <Pine.LNX.4.64.0602012246330.3680@hermes-2.csi.cam.ac.uk> (raw)
In-Reply-To: <20060201220903.GE16471@esmail.cup.hp.com>
On Wed, 1 Feb 2006, Grant Grundler wrote:
> On Wed, Feb 01, 2006 at 01:41:03PM -0800, Chen, Kenneth W wrote:
> > > Well, if it doesn't matter, why is unsigned int better?
> >
> > I was coming from the angle of having bitop operate on unsigned
> > int *, so people don't have to type cast or change bit flag variable
> > to unsigned long for various structures. With unsigned int type for
> > bit flag, some of them are not even close to fully utilized. for example:
> >
> > thread_info->flags uses 18 bits
> > thread_struct->flags uses 7 bits
> >
> > It's a waste of memory to define a variable that kernel will *never*
> > touch the 4 MSB in that field.
>
> Agreed. Good point. But this can be mitigated if the code using "unsigned int"
> (or unsigned byte) first loads the value into a local unsigned long variable.
> That typically translates into a tmp register anyway. Compiler will help
> you find places where that needs to happen.
>
> Counter point is bit arrays (e.g. bit maps) like cpumask_t are
> typically much larger than 32-bits (typically distro's ship with
> NR_CPUS set to 256 or so). File system code also likes bit arrays
> for block allocation tables. Searching a bit array using unsigned
> long is 2x faster on 64-bit architectures. I don't want to give
> that up and I'm pretty sure Tony Luck, Paul Mckerras and a few
> others would object unless you can give a better reason.
Err, searching by anything other than bytes is useless for a file system
driver. Otherwise you get all sorts of disgustingly horrible allocation
patterns depending on the endianness of the machine...
Best regards,
Anton
--
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/
next prev parent reply other threads:[~2006-02-01 22:49 UTC|newest]
Thread overview: 82+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-01-25 11:26 [PATCH 0/6] RFC: use include/asm-generic/bitops.h Akinobu Mita
2006-01-25 11:28 ` [PATCH 1/6] {set,clear,test}_bit() related cleanup Akinobu Mita
2006-01-25 11:46 ` Andi Kleen
2006-01-26 16:14 ` Pavel Machek
2006-01-26 16:47 ` Russell King
2006-01-26 19:14 ` Paul Jackson
2006-01-25 11:30 ` [PATCH 2/6] use non atomic operations for minix_*_bit() and ext2_*_bit() Akinobu Mita
2006-01-25 11:32 ` [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h Akinobu Mita
2006-01-25 11:54 ` Keith Owens
2006-01-26 2:13 ` Akinobu Mita
2006-01-26 2:19 ` Akinobu Mita
2006-01-25 20:02 ` Russell King
2006-01-25 20:59 ` Grant Grundler
2006-01-26 3:27 ` Akinobu Mita
2006-01-26 3:29 ` [PATCH 1/12] generic *_bit() Akinobu Mita
2006-02-01 15:11 ` Chen, Kenneth W
2006-02-01 18:02 ` Christoph Hellwig
2006-02-01 18:07 ` Chen, Kenneth W
2006-02-01 19:19 ` Russell King
2006-02-01 19:25 ` Chen, Kenneth W
2006-02-01 19:35 ` Russell King
2006-02-03 10:24 ` Geert Uytterhoeven
2006-02-03 10:27 ` Russell King
2006-02-01 19:39 ` Grant Grundler
2006-02-01 21:41 ` Chen, Kenneth W
2006-02-01 22:09 ` Grant Grundler
2006-02-01 22:49 ` Anton Altaparmakov [this message]
2006-02-02 0:08 ` Grant Grundler
2006-02-02 8:52 ` Anton Altaparmakov
2006-02-02 10:13 ` Andreas Schwab
2006-02-02 22:43 ` Paul Mackerras
2006-01-26 3:30 ` [PATCH 2/12] generic __ffs() Akinobu Mita
2006-01-26 3:31 ` [PATCH 3/12] generic ffz() Akinobu Mita
2006-01-26 8:21 ` Michael Tokarev
2006-01-27 6:39 ` [PATCH] parisc: add ()-pair in __ffs() Akinobu Mita
2006-01-26 3:32 ` [PATCH 4/12] generic fls() and fls64() Akinobu Mita
2006-01-26 3:33 ` [PATCH 5/12] generic find_{next,first}{,_zero}_bit() Akinobu Mita
2006-01-26 3:34 ` [PATCH 6/12] generic sched_find_first_bit() Akinobu Mita
2006-01-26 3:35 ` [PATCH 7/12] generic ffs() Akinobu Mita
2006-01-26 3:36 ` [PATCH 8/12] generic hweight{32,16,8}() Akinobu Mita
2006-01-26 7:12 ` Balbir Singh
2006-01-26 10:04 ` Rutger Nijlunsing
2006-01-27 4:55 ` Akinobu Mita
2006-01-27 5:40 ` Balbir Singh
2006-01-27 6:40 ` Akinobu Mita
2006-01-31 11:14 ` Balbir Singh
2006-01-26 18:57 ` Bryan O'Sullivan
2006-01-27 4:43 ` Akinobu Mita
2006-01-27 5:23 ` Bryan O'Sullivan
2006-01-26 3:36 ` [PATCH 9/12] generic hweight64() Akinobu Mita
2006-01-26 7:05 ` Balbir Singh
2006-01-26 3:38 ` [PATCH 10/12] generic ext2_{set,clear,test,find_first_zero,find_next_zero}_bit() Akinobu Mita
2006-01-26 3:38 ` [PATCH 11/12] generic ext2_{set,clear}_bit_atomic() Akinobu Mita
2006-01-26 3:39 ` [PATCH 12/12] generic minix_{test,set,test_and_clear,test,find_first_zero}_bit() Akinobu Mita
2006-01-25 23:25 ` [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h Ian Molton
2006-01-26 0:06 ` Richard Henderson
2006-01-26 4:34 ` Edgar Toernig
2006-01-26 17:30 ` Richard Henderson
2006-01-26 8:55 ` Russell King
2006-01-26 16:18 ` [parisc-linux] " Grant Grundler
2006-01-26 16:30 ` Nicolas Pitre
2006-01-26 16:40 ` Russell King
2006-01-26 23:04 ` Grant Grundler
2006-01-26 23:03 ` Russell King
2006-01-29 7:12 ` Stuart Brady
2006-01-30 4:03 ` David S. Miller
2006-01-30 17:06 ` Ralf Baechle
2006-01-30 19:50 ` Stuart Brady
2006-01-30 23:02 ` David S. Miller
2006-01-27 0:28 ` [parisc-linux] Re: [PATCH 3/6] C-language equivalents of John David Anglin
2006-01-27 12:51 ` [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h Hirokazu Takata
2006-01-30 3:29 ` Akinobu Mita
2006-01-25 11:34 ` [PATCH 5/6] fix warning on test_ti_thread_flag() Akinobu Mita
2006-01-25 12:28 ` Geert Uytterhoeven
2006-01-25 22:28 ` Paul Mackerras
2006-01-26 0:04 ` David S. Miller
2006-01-25 11:35 ` [PATCH 6/6] remove unused generic bitops in include/linux/bitops.h Akinobu Mita
[not found] ` <20060125113336.GE18584@miraclelinux.com>
2006-01-26 1:49 ` [PATCH 4/6] use include/asm-generic/bitops for each architecture Akinobu Mita
2006-01-26 2:37 ` Grant Grundler
2006-01-27 13:04 ` Hirokazu Takata
2006-01-30 3:15 ` Akinobu Mita
2006-02-03 17:07 [PATCH 1/12] generic *_bit() Luck, Tony
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