Re: [dpdk-users] Performance of rte_eth_stats_get

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 19 May 2021 15:14:38 +0000
"Van Haaren, Harry" <harry.van.haaren@intel.com> wrote:

> > -----Original Message-----
> > From: users <users-bounces@dpdk.org> On Behalf Of Filip Janiszewski
> > Sent: Wednesday, May 19, 2021 2:10 PM
> > To: users@dpdk.org
> > Subject: [dpdk-users] Performance of rte_eth_stats_get
> > 
> > Hi,
> > 
> > Is it safe to call rte_eth_stats_get while capturing from the port?
> > 
> > I'm mostly concerned about performance, if rte_eth_stats_get will in any
> > way impact the port performance, in the application I plan to call the
> > function from a thread that is not directly involved in the capture,
> > there's another worker responsible for rx bursting, but I wonder if the
> > NIC might get upset if I call it too frequently (say 10 times per
> > second) and potentially cause some performance issues.
> > 
> > The question is really Nic agnostic, but if the Nic vendor is actually
> > relevant then I'm running Intel 700 series nic and Mellanox ConnectX-4/5.  
> 
> To understand what really goes on when getting stats, it might help to list the
> steps involved in getting statistics from the NIC HW.
> 
> 1) CPU sends an MMIO read (Memory Mapped I/O, aka, sometimes referred
> to as a "pci read") to the NIC.
> 2) The PCI bus has to handle extra TLPs (pci transactions) to satisfy read
> 3) NIC has to send a reply based on accessing its internal counters
> 4) CPU gets a result from the PCI read.
> 
> Notice how elegantly this whole process is abstracted from SW? In code, reading
> a stat value is just dereferencing a pointer that is mapped to the NIC HW address.
> In practice from a CPU performance point of view, doing an MMIO-read is one of
> the slowest things you can do. You say the stats-reads are occurring from a thread
> that is not handling rx/datapath, so perhaps the CPU cycle cost itself isn't a concern.
> 
> Do note however, that when reading a full set of extended stats from the NIC, there
> could be many 10's to 100's of MMIO reads (depending on the statistics requested,
> and how the PMD itself is implemented to handle stats updates).
> 
> The PCI bus does become more busy with reads to the NIC HW when doing lots of
> statistic updates, so there is some more contention/activity to be expected there.
> The PCM tool can be very useful to see MMIO traffic, you could measure how many
> extra PCI transactions are occurring due to reading stats every X ms?
> https://github.com/opcm/pcm
> 
> I can recommend measuring pkt latency/jitter as a histogram, as then outliers in performance
> can be identified. If you specifically want to identify if these are due stats reads, compare
> with a "no stats reads" latency/jitter histogram, and graphically see the impact.
> In the end if it doesn't affect packet latency/jitter, then it has no impact right?
> 
> Ultimately, I can't give a generic answer - best steps are to measure carefully and find out!
> 
> > Thanks  
> 
> Hope the above helps and doesn't add confusion :)  Regards, -Harry

Many drivers require transactions with the firmware via mailbox.
And that transaction needs a spin wait for the shared area.