Re: [dpdk-dev] [PATCH v2] pipeline: add statistics for librte_pipeline ports and tables

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2015-05-20 23:41, Dumitrescu, Cristian:
> From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> > 2015-05-20 13:57, Dumitrescu, Cristian:
> > > From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> > > > Thanks for the detailed explanation.
> > > >
> > > > You are raising a trade-off problem about
> > > > feature/maintenance/performance.
> > > > I think we must choose among 4 solutions:
> > > > 	1/ always enabled
> > > > 	2/ build-time log level
> > > > 	3/ run-time option
> > > > 	4/ build-time option
> > > >
> > > > It's good to have this discussion and let other contributors giving their
> > > > opinion.
> > > >
> > > > 2015-05-19 22:41, Dumitrescu, Cristian:
> > > > > 1. What is the technical solution to avoid performance loss due to stats
> > > > > support?
> > > > > Generally, I would agree with you that config options should be avoided,
> > > > > especially those that alter the API (function prototypes, data structure
> > > > > layouts, etc). Please note this is not the case for any of our patches,
> > > > > as only the stats collection is enabled/disabled, while the data
> > > > > structures and functions are not changed by the build time option.
> > > > >
> > > > > But what can you do for statistics? We all know that collecting the stats
> > > > > sucks up CPU cycles, especially due to memory accesses, so stats always
> > > > > have a run-time cost. Traditionally, stats are typically enabled for
> > > > > debugging purposes and then turned off for the release version when
> > > > > performance is required. How can this be done if build time flags are
> > > > > not used?
> > > >
> > > > Statistics in drivers are always enabled (first solution).
> > > > If those statistics are only used for debugging, why not using the build-
> > > > time option CONFIG_RTE_LOG_LEVEL? (second solution)
> > >
> > > Can you please describe what solution 2 on your list (build-time log
> > > level) consists of?
> > >
> > > I see log level useful for printing messages when an event takes place,
> > > but this is not what these stats patches are about. We want to poll
> > > for those counters on demand: if the build-time flag is off, then the
> > > value of those counters is 0; when the build-time is on, then the stats
> > > counters have the real value. Basically, the build-time flag only
> > > enables/disables the update of the counters at run-time, which is where
> > > CPU cycles are consumed. I am not sure how the log levels can help here?
> > 
> > I think that counting stats is a kind of logging.
> > Some stats are always counted (see drivers) and you want to use these ones
> > only for debugging (after rebuilding DPDK with some debug options).
> > So I suggest, as second solution, to check CONFIG_RTE_LOG_LEVEL is at
> > debug
> > level instead of having one option per module.
> > It would be implemented with "#if RTE_LOG_LEVEL == RTE_LOG_DEBUG" in
> > RTE_PIPELINE_STATS_ADD.
> 
> The problem I see with this approach is that you cannot turn off debug
> messages while still having the statistics collected.  We should allow
> people to collects the stats without getting the debug messages. How do
> we do this?

By setting build-time log level to debug, you would enable stats and debug
messages. Then you can adjust the log messages level with the run-time
option --log-level.