Re: librte_pmd_ixgbe implementation of ixgbe_dev_rx_queue_count

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 29 Mar 2016 10:31:19 +0100
Bruce Richardson <bruce.richardson@intel.com> wrote:

> On Mon, Mar 28, 2016 at 06:45:26PM -0700, Mohammad El-Shabani wrote:
> > Hi,
> > Looking into why it hurts performance, I see that ixgbe_dev_rx_queue_count
> > is implemented a scan of elements of rx descriptors, which is very
> > expensive. I am wondering why its implemented the way it is. Could it not
> > just read the head location from the driver?
> > 
> > Thanks!
> > Mohammad El-Shabani
> 
> It's likely that reading the head location from the driver will be even slower
> than scanning the descriptor rings in memory. Access to PCI is very much slower
> than accessing memory - especially since on platforms with DDIO, many memory
> accesses will actually be cache reads.
> 
> That being said, I haven't actually written a test to prove this out, so feel
> free to try out the head pointer read method instead and see if it improves
> things. The results may vary depending on how far ahead needs to be scanned,
> but certainly for the empty ring case, the descriptor scan method will be far
> faster than a head read.
> 
> Regards,
> /Bruce

Also the most common use case is "is there any more packets ready before
I go to sleep on epoll", and the descriptor done API tells more than
is needed.