Thanks, that helps a lot. By the way - how do strings fit in this? Is it best that they are last? Maybe best they are first being the largest?
Amit Margalit IBM XIV - Storage Reinvented XIV-NAS Development Team Tel. 03-689-7774 Fax. 03-689-7230 From: Mathieu Desnoyers <[email protected]> To: Amit Margalit/Israel/IBM@IBMIL Cc: [email protected] Date: 07/10/2013 08:25 PM Subject: Re: [lttng-dev] Question about performance and tracepoint structure * Amit Margalit ([email protected]) wrote: > Hi, > > This may be a bit of a noob question, and I think I read about this > somewhere but I can't find it now. > > When I declare a tracepoint that's made up of a uint8_t, followed by a > uint16_t followed by a uint32_t, for example, there are 2 ways to handle > this - packed or unpacked. > > If we're doing packed, then we have to include some code that does the > packing properly as alignment issues can creep up. This code causes us to > incur some CPU cycles at that point. it's not code. it's just that cpus are slightly slower at doing unaligned accesses than aligned accesses. > If we're doing unpacked, then we incur some cycles on passing the event to > the sessiond, and sessiond suffers by having to write down some extra > bytes, which is less efficient. yes > > Either way, I think it means I can gain some minor performance > improvements if I reorder my tracepoint arguments to a more efficient > order - alignment-wise. Well, yes. You will want to reorder the tracepoint "TP_FIELDS" so the largest fields come first, followed by the smaller fields. The order of arguments (TP_ARGS) does not matter. Thanks, Mathieu > > Am I barking up the wrong tree here? > > Thanks, > > Amit Margalit > IBM XIV - Storage Reinvented > XIV-NAS Development Team > Tel. 03-689-7774 > Fax. 03-689-7230 > _______________________________________________ > lttng-dev mailing list > [email protected] > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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