From: Linus Torvalds <torvalds@osdl.org>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Russell King <rmk+lkml@arm.linux.org.uk>,
Thomas Gleixner <tglx@linutronix.de>,
Greg Weeks <greg.weeks@timesys.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2.4] jffs2 aligment problems
Date: Mon, 7 Jun 2004 13:54:08 -0700 (PDT) [thread overview]
Message-ID: <Pine.LNX.4.58.0406071351450.1637@ppc970.osdl.org> (raw)
In-Reply-To: <1086640771.29255.57.camel@localhost.localdomain>
On Mon, 7 Jun 2004, David Woodhouse wrote:
>
> On Mon, 2004-06-07 at 12:22 -0700, Linus Torvalds wrote:
> > I don't see it as a correctness issue, I see it as a performance issue.
>
> In the case in question it's very much _not_ a performance issue. We're
> writing a buffer to FLASH memory. The time it takes to read the word
> from RAM is entirely lost in the noise compared with the time it takes
> to write it to the flash.
Not if you have to take an alignment fault, which is easily several
thousand cycles.
Think of "get_unaligned()" as a worst-case limiter. It can make the best
case be worse on architectures where it matters, but it can make the worst
case go from thousands of cycles to just single cycles.
And your flash isn't _that_ slow. Thousands of cycles that can't even
overlap with any flash IO _does_ show up.
Now, whether the unaligned case is common enough for people to even worry,
I don't know.
Linus
next prev parent reply other threads:[~2004-06-07 20:54 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-06-07 15:08 Greg Weeks
2004-06-07 15:36 ` Thomas Gleixner
2004-06-07 16:03 ` Linus Torvalds
2004-06-07 16:41 ` Russell King
2004-06-07 19:14 ` David Woodhouse
2004-06-07 19:22 ` Linus Torvalds
2004-06-07 20:39 ` David Woodhouse
2004-06-07 20:54 ` Linus Torvalds [this message]
2004-06-07 20:56 ` Thomas Gleixner
2004-06-07 21:29 ` David Woodhouse
2004-06-07 21:40 ` Thomas Gleixner
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