linux-kernel - Re: [PATCH v6 1/6] perf header: Record first sample time and last sample time in perf file header

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Date:   Wed, 22 Nov 2017 08:55:06 +0800
From:   "Jin, Yao" <yao.jin@...ux.intel.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     jolsa@...nel.org, peterz@...radead.org, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, Linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, kan.liang@...el.com, yao.jin@...el.com
Subject: Re: [PATCH v6 1/6] perf header: Record first sample time and last
 sample time in perf file header



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