Re: [PATCH v7 0/2]: perf: reduce data loss when profiling highly parallel CPU bound workloads

[Jiri Olsa]

On Wed, Sep 05, 2018 at 10:16:42AM +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.
> 
> Applying asynchronous trace streaming thru Posix AIO API
> (http://man7.org/linux/man-pages/man7/aio.7.html) 
> lowers data loss metrics value providing 2x improvement -
> lowering 98% loss to almost 0%.
> 
> ---
>  Alexey Budankov (2):
>         perf util: map data buffer for preserving collected data
> 	perf record: enable asynchronous trace writing
>  
>  tools/perf/builtin-record.c | 197 +++++++++++++++++++++++++++++++++++++++++++-
>  tools/perf/perf.h           |   1 +
>  tools/perf/util/evlist.c    |   7 +-
>  tools/perf/util/evlist.h    |   3 +-
>  tools/perf/util/mmap.c      | 110 +++++++++++++++++++++----
>  tools/perf/util/mmap.h      |  10 ++-
>  6 files changed, 302 insertions(+), 26 deletions(-)
> 
> ---
>  Changes in v7:
>  - implemented handling record.aio setting from perfconfig file

can't apply this version on Arnaldo's perf/core...

[jolsa@krava linux-perf]$ git am /tmp/ab/
Applying: perf util: map data buffer for preserving collected data
error: patch failed: tools/perf/util/mmap.c:166
error: tools/perf/util/mmap.c: patch does not apply

thanks,
jirka