Re: Ethdev tracepoints optimization

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Aug 19, 2024 at 05:07:18PM +0530, Jerin Jacob wrote:
> On Mon, Aug 19, 2024 at 4:13 PM Ferruh Yigit <ferruh.yigit@amd.com> wrote:
> >
> > On 8/15/2024 8:32 PM, Adel Belkhiri wrote:
> > > Hi DPDK Community,
> > >
> > > I am currently working on developing performance analyses for
> > > applications using the ethdev library. These analyses are being
> > > implemented in Trace Compass, an open-source performance analyzer. One
> > > of the views I’ve implemented shows the rate of traffic received or sent
> > > by an ethernet port, measured in packets per second. However, I've
> > > encountered an issue with the lib.ethdev.rx.burst event, which triggers
> > > even when no packets are polled, leading to a significant number of
> > > irrelevant events in the trace. This becomes problematic as these
> > > "empty" events can overwhelm the tracer buffer, potentially causing the
> > > loss of more critical events due to their high frequency.
> > >
> > > To address this, I've modified the DPDK code in lib/ethdev/rte_ethdev.h
> > > to add a conditional statement that only triggers the event when nb_rx >
> > > 0. My question to the community is whether there are use cases where an
> > > "empty" lib.ethdev.rx.burst event could be useful. If not, would there
> > > be interest in submitting a patch with this modification?
> > >
> >
> > Tracepoint is good way to get frequency of the calls, so I believe there
> > can be value of getting empty burst calls too.
> >
> > But your usecase also a valid one. I wonder if it works to have separate
> > trace calls, for empty and non-empty ones, and how this additional
> > branching impacts the performance, at least branch should be wrapped
> > with 'RTE_ENABLE_TRACE_FP' macro to not impact non-tracing usage.
> 
> 
> CTF(Common trace format) post processing tools can check the value for
> each field and timestap.
> So it will easy to skip the ones with no packet for this case.
>
The trouble with empty polls is that they are very, very fast, so if no
traffic is arriving you can quickly fill your entire trace buffer with
nothing but empty polls. I believe that is the situation the original
poster wanted to avoid.

/Bruce