* David S. Miller [ref] 2005-07-05 16:45

From: Thomas Graf <tgraf@suug.ch>
Date: Wed, 6 Jul 2005 01:41:04 +0200

quoted

I still think we can fix this performance issue without manually
unrolling the loop or we should at least try to. In the end gcc
should notice the constant part of the loop and move it out so
basically the only difference should the additional prio++ and
possibly a failing branch prediction.

But the branch prediction is where I personally think a lot
of the lossage is coming from.  These can cost upwards of 20
or 30 processor cycles, easily.  That's getting close to the
cost of a L2 cache miss.

Absolutely. I think what happens is that we produce predicion
failures due to the logic within qdisc_dequeue_head(), I
cannot back this up with numbers though.

I see the difficulties with this change now, why don't we revisit
this some time in the future?

Fine with me.

Eric, the patch I just posted should result in the same branch
prediction as your loop unrolling. The only additional overhead
we still have is the list + prio thing and an additional conditional
jump to do the loop. If you have the cycles etc. it would be nice
to compare it with your numbers.