Re: bzero & bcopy alignment

2007/1/22, Luigi Rizzo <rizzo@xxxxxxxx>:
On Sun, Jan 21, 2007 at 11:25:14PM +0100, Ivan Voras wrote:
> Luigi Rizzo wrote:
> > On Sun, Jan 21, 2007 at 10:41:09PM +0100, Ivan Voras wrote:
> >> Following recent discussion on alignment of bzero() and bcopy(), I've
> >> added some statistics collecting code to bzero() and bcopy() for
> >> practice (on a RELENG_6 box), and here are the cumulative results for
> >> argument alignment:
> >
> > i think these profiles depend heavily on the hardware
> > and usage patterns.
>
> Yes, I agree. For what it's worth, this was on vmware, almost no network
> activity.
>
> > e.g. some network drivers force you to aligned buffers
> > which results in misaligned payload requesting in
> > turn an unaligned bcopy. Not that one can help with this,
> > but i think that is also important to locate the locations
> > in the source where the poorly aligned (1-2, maybe
> > 4 and 8 to some degree) ops occur.
>
> Any magic tricks to identify the caller of "current" function in the kernel?

i suppose i would remap those function as

#define bcopy(a, b, c) \
do { \
track_bcopy(__PRETTY_FUNCTION__, __FILE__, __LINE__, a, b, c);
original_bcopy(a, b, c);
}

and the same for bzero, and then use function track_bcopy()
to simply record the instances in some data structure that you
can then export in some way to userland for stats gathering.
In the simplest form you could e.g. filter unaligned accesses
and just dump them to a buffer exported via sysctl;
if you want to make things smarter/more efficient, you could
hash __FILE__, __LINE__ to avoid duplicates (and probably,
because these are constant strings, you can just compare
the string pointers without having to do a full strcmp).

A better approach (at least for ia32, I'm not aware what is the
situation on the other architectures) would be using the vectorized
entry for this family of function (i386/include/md_var.h).

Attilio


--
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