On 12/27/2013 09:48 AM, Steve Underwood wrote:
On 12/27/2013 05:23 PM, Gordan Bobic wrote:
On 12/27/2013 08:32 AM, Richard W.M. Jones wrote:
On Thu, Dec 26, 2013 at 07:49:58PM +0000, Gordan Bobic wrote:
On 12/26/2013 02:20 PM, Derek Atkins wrote:
Gordan Bobic <gordan@xxxxxxxxxx> writes:
(e.g. (but not limited to) a large number of packages make little or
no effort to ensure memory accesses are aligned - including the likes
of e2fsprogs, and transparent alignment fixup in hardware is only
available on armv7 and later).
I'm surprised that Ted isn't willing to fix issues in e2fsprogs.
If you can point me to the upstream bug reports I can ping him to see
what's up?
Take a look here:
http://comments.gmane.org/gmane.comp.file-systems.ext4/33324
As has been mentioned before, there is a whole shedload of packages
that have similar issues - I have seen literally thousands of
alignment faults get reported (I have the alignment set to fix+warn
on my armv5tel builders) in various packages during build and test
stages. Once upon a time I planned to collate the data and get the
issue reported to all upstream maintainers, but that is a mammoth
task just to report, let alone fix, and I have very little faith
there is enough will among the developers to fix all the affected
packages and ensure they write code that isn't affected by this
problem going forward.
I disagree that it is even a problem; except in a very small number of
cases where it causes a measurable slowdown.
It's a more philosophical issue - since alignment issues arguably
arise from poor programming practice in the first place, should there
be pressure to not produce code that suffers from such issues?
>
If you make an x86 machine work through alignment issues in software,
rather than let the hardware sort it out, you will pay a speed penalty.
How is transparent alignment fixup going to give you back the
performance you lose from accesses straddling cache lines?
In a lot of protocol code misalignment is forced upon you by the
protocol, and sorting it out in software can lead to a *considerable*
speed penalty.
Fair, I can see this being one case where alignment auto-fixup is
beneficial, but how many commonly used protocols are there where this is
actually a problem? Has anyone ever assessed this comprehensively? Is
there a list somewhere?
Gordan
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