bzero & bcopy alignment

[Attilio Rao]

2007/1/22, Luigi Rizzo <rizzo at icir.org>:
> 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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