On Wed, 20 May 2015 16:44:35 +0200 Thomas Monjalon <thomas.monjalon at 6wind.com> wrote:
> Please Cristian, do not top post. > I'm replacing your comment in the right context. > > 2015-05-20 13:57, Dumitrescu, Cristian: > > From: Thomas Monjalon [mailto:thomas.monjalon at 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. > In my experience, per-queue or per-cpu statistics have no visible performance impact. And every real world deployment wants statistics
