linux-kernel - Re: [PATCH v3 0/2]: perf: reduce data loss when profiling highly parallel CPU bound workloads

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Date:   Tue, 28 Aug 2018 10:59:04 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 0/2]: perf: reduce data loss when profiling highly
 parallel CPU bound workloads

On Mon, Aug 27, 2018 at 08:03:21PM +0300, Alexey Budankov wrote:
> 
> Currently in record mode the tool implements trace writing serially. 
> The algorithm loops over mapped per-cpu data buffers and stores ready 
> data chunks into a trace file using write() system call.
> 
> At some circumstances the kernel may lack free space in a buffer 
> because the other buffer's half is not yet written to disk due to 
> some other buffer's data writing by the tool at the moment.
> 
> Thus serial trace writing implementation may cause the kernel 
> to loose profiling data and that is what observed when profiling 
> highly parallel CPU bound workloads on machines with big number 
> of cores.
> 
> Experiment with profiling matrix multiplication code executing 128 
> threads on Intel Xeon Phi (KNM) with 272 cores, like below,
> demonstrates data loss metrics value of 98%:
> 
> /usr/bin/time perf record -o /tmp/perf-ser.data -a -N -B -T -R -g \
>     --call-graph dwarf,1024 --user-regs=IP,SP,BP \
>     --switch-events -e cycles,instructions,ref-cycles,software/period=1,name=cs,config=0x3/Duk -- \
>     matrix.gcc
> 
> Data loss metrics is the ratio lost_time/elapsed_time where 
> lost_time is the sum of time intervals containing PERF_RECORD_LOST 
> records and elapsed_time is the elapsed application run time 
> under profiling.

I like the idea and I think it's good direction to go, but could
you please share some from perf stat or whatever you used to meassure
the new performance?

thanks,
jirka