From: Theodore Ts'o <tytso@mit.edu>
To: Peter Fordham <peter.fordham@gmail.com>
Cc: linux-ext4@vger.kernel.org
Subject: Re: spinlocks in ext4
Date: Mon, 8 Oct 2012 20:54:25 -0400 [thread overview]
Message-ID: <20121009005425.GB20682@thunk.org> (raw)
In-Reply-To: <CAK5=ZRLQErM1dOT_Tuf-SC4-3VK_o6TRg-0k8A=iQGgFY6S+aw@mail.gmail.com>
On Mon, Oct 08, 2012 at 04:33:45PM -0700, Peter Fordham wrote:
>
> Can someone give me a quick outline of why spinlocks are required in
> the EXT4 code? Don't all file-system requests originate from user
> context, hence meaning all locking be done with mutexes or semaphores.
Mutexes are incredibly expensive in the contended case, since you
basically have to take a trip through the scheduler. If the other CPU
is only going to be holding the lock for a few dozen cycles, a
spinlock is far preferable to a mutex.
> I'm doing some profiling on an ARM device it's showing up spin unlock
> taking a lot of time and I'd like to migrate to using mutex's instead
> since they don't incur penalties from synchronization instructions
> like DMB. I'm guessing there's some underlying reason why this isn't
> safe and I'd like to understand it.
Why in the world does ARM have expensive spinlocks? ARM64 is *doomed*
if this is a fundamental property of the ARM processor design...
- Ted
next prev parent reply other threads:[~2012-10-09 0:54 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-08 23:33 spinlocks in ext4 Peter Fordham
2012-10-09 0:54 ` Theodore Ts'o [this message]
2012-10-09 1:10 ` Theodore Ts'o
2012-10-11 18:37 ` Peter Fordham
2012-10-12 0:17 ` Theodore Ts'o
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