From: "Jon Medhurst (Tixy)" <tixy@linaro.org>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Huang Shijie <shijie.huang@arm.com>,
Joerg Roedel <joro@8bytes.org>,
will.deacon@arm.com, iommu@lists.linux-foundation.org,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is NULL
Date: Wed, 20 Jan 2016 16:00:01 +0000 [thread overview]
Message-ID: <1453305601.3417.12.camel@linaro.org> (raw)
In-Reply-To: <569F9DCA.3030808@arm.com>
On Wed, 2016-01-20 at 14:46 +0000, Robin Murphy wrote:
> > Of course, the 0.004014s maybe not accurate enough, it is just an
> > approximate number.
>
> A mean and standard deviation of at least, say, 5 runs each with and
> without the patch would be considerably more meaningful (even if
> still
> far from statistically significant).
It wouldn't surprise me if replacing the proposed change with an 'asm
volatile("nop")' or two also give a boot time delta of several
milliseconds (due to change in cache line alignment of functions). I
don't believe you can reliably measure such minor changes.
It doesn't mean that the proposed change isn't a good addition though,
it obviously results in less code getting executed for the cost of one
or two instructions for a compare and branch.
--
Tixy
WARNING: multiple messages have this Message-ID (diff)
From: tixy@linaro.org (Jon Medhurst (Tixy))
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is NULL
Date: Wed, 20 Jan 2016 16:00:01 +0000 [thread overview]
Message-ID: <1453305601.3417.12.camel@linaro.org> (raw)
In-Reply-To: <569F9DCA.3030808@arm.com>
On Wed, 2016-01-20 at 14:46 +0000, Robin Murphy wrote:
> > Of course, the 0.004014s maybe not accurate enough, it is just an
> > approximate number.
>
> A mean and standard deviation of at least, say, 5 runs each with and
> without the patch would be considerably more meaningful (even if
> still
> far from statistically significant).
It wouldn't surprise me if replacing the proposed change with an 'asm
volatile("nop")' or two also give a boot time delta of several
milliseconds (due to change in cache line alignment of functions). I
don't believe you can reliably measure such minor changes.
It doesn't mean that the proposed change isn't a good addition though,
it obviously results in less code getting executed for the cost of one
or two instructions for a compare and branch.
--
Tixy
next prev parent reply other threads:[~2016-01-20 16:00 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-12 2:15 [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is NULL Huang Shijie
2016-01-12 2:15 ` Huang Shijie
2016-01-12 2:15 ` Huang Shijie
[not found] ` <1452564905-2662-1-git-send-email-shijie.huang-5wv7dgnIgG8@public.gmane.org>
2016-01-20 12:02 ` Joerg Roedel
2016-01-20 12:02 ` Joerg Roedel
2016-01-20 12:02 ` Joerg Roedel
[not found] ` <20160120120225.GD18805-zLv9SwRftAIdnm+yROfE0A@public.gmane.org>
2016-01-20 13:34 ` Huang Shijie
2016-01-20 13:34 ` Huang Shijie
2016-01-20 13:34 ` Huang Shijie
[not found] ` <20160120133401.GA3487-bi+AKbBUZKY6gyzm1THtWbp2dZbC/Bob@public.gmane.org>
2016-01-20 14:46 ` Robin Murphy
2016-01-20 14:46 ` Robin Murphy
2016-01-20 14:46 ` Robin Murphy
2016-01-20 16:00 ` Jon Medhurst (Tixy) [this message]
2016-01-20 16:00 ` Jon Medhurst (Tixy)
[not found] ` <569F9DCA.3030808-5wv7dgnIgG8@public.gmane.org>
2016-01-21 2:04 ` Huang Shijie
2016-01-21 2:04 ` Huang Shijie
2016-01-21 2:04 ` Huang Shijie
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