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

From: Jiri Olsa
Date: Tue Aug 28 2018 - 04:59:09 EST

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/ -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?