Messages in this thread | | | Subject | Re: [PATCH v6 1/6] perf header: Record first sample time and last sample time in perf file header | From | "Jin, Yao" <> | Date | Wed, 22 Nov 2017 08:55:06 +0800 |
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On 11/22/2017 12:38 AM, Arnaldo Carvalho de Melo wrote: > Em Mon, Nov 06, 2017 at 08:21:06PM +0800, Jin Yao escreveu: >> perf report/script/... have a --time option to limit the time range >> of output. That's very useful to slice large traces, e.g. when processing >> the output of perf script for some analysis. >> >> But right now --time only supports absolute time. Also there is no fast >> way to get the start/end times of a given trace except for looking at it. >> This makes it hard to e.g. only decode the first half of the trace, which >> is useful for parallelization of scripts >> >> Another problem is that perf records are variable size and there is no >> synchronization mechanism. So the only way to find the last sample reliably >> would be to walk all samples. But we want to avoid that in perf report/... >> because it is already quite expensive. That is why storing the first sample >> time and last sample time in perf record is better. >> >> This patch creates a new header feature type HEADER_SAMPLE_TIME and related >> ops. Save the first sample time and the last sample time to the feature >> section in perf file header > > I'll add a clarification here, right after your text: > > ". That will be done when, for instance, processing build-ids, where we > already have to process all samples to create the build-id table, take > advantage of that to further amortize that processing by storing > HEADER_SAMPLE_TIME to make 'perf report/script' faster when using > --time." >
Thanks Arnaldo! The clarification is good!
Thanks Jin Yao
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