Re: [9fans] etherigbe.c using _xinc?

[Russ Cox <rsc@swtch.com>]

On Tue, Dec 8, 2009 at 4:32 PM, erik quanstrom <quanstro@quanstro.net> wrote:
>> but the former does two operations and the latter
>> only one.  your claim was that _xinc is slower
>> than incref (== lock(), x++, unlock()).  but you are
>> timing xinc+xdec against incref.
>
> sure.  i was looking it as a kernel version of a
> semaphore.

no, your original claim was that incref/decref
was faster than _xinc/_xdec.  the numbers
don't support that claim.

> the reference
> counting is a heavy price to pay on every network
> block, when it is only used by ip/gre.c.

has the network gotten fast enough that an extra
bus transaction per block slows it down?
it seems like gigabit ethernet would be around
100k packets per second, so the extra 50ns
or so per packet would be 5ms per second in
practice, which is significantly but hardly
seems prohibitive.

> before allocb/freeb
> did 2 lock/unlocks.  now it does 2 unlock/locks
> + 2 xinc/xdec, and is, in the best case 31% slower.
> and in the worst case 90% slower.

i don't know how you get those numbers but
anything even approaching that would mean that
the kernel is spending all its time in igberballoc,
at which point you probably have other things
to fix.

russ