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?
Is there a way to find
out if a program is doing an excessive number of alignment fixups?
Mostly - run with fix+warn
# echo 3 > /proc/cpu/alignment
and keep an eye on syslog (or get logwatch to do so for you).
If you have a relatively simple setup (basic LAMP server and little
else), hardly anything gets logged. If you have a koji/mock farm, the
syslog is flooded with the warnings. I haven't actually measured this,
but my impression is that a non-trivial fraction of the alignment
warnings actually occur in the test suites for various packages that
support them.
Basically this is an architectural problem in ARM, and not something
developers should go through hoops to fix except in the tiny number of
cases where it causes an actual, measurable problem.
There are always performance drawbacks to not paying attention to
alignment, since unaligned access also end up straddling cache lines,
which also comes with a performance hit, even when there is transparent
alignment auto-fixup in hardware.
IIRC, SPARC and Itanium also have, or at least had issues with unaligned
accesses. I don't know if they introduced a transparent fixup in
hardware since.
It's also not a case of jumping through hoops, it's a matter of not
using poor practices such as allocating arrays of char for buffers and
then casting them into structs.
Gordan
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