Re: [PATCH v15 0/3]: perf: reduce data loss when profiling highly parallel CPU bound workloads

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Nov 06, 2018 at 11:53:02AM +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 [1] lowers 
> data loss metrics value providing 2x improvement (from 98% to ~1%)
> 
> Asynchronous trace streaming is currently limited to glibc linkage.
> musl libc [5] also provides Posix AIO API implementation, however 
> the patchkit is not tested with it. There may be other libc libraries 
> linked by Perf tool that currently lack Posix AIO API support [2], 
> [3], [4] so NO_AIO define may be used to limit Perf tool binary to 
> serial streaming only.
> 
> ---
>  Alexey Budankov (3):
> 	perf util: map data buffer for preserving collected data
> 	perf record: enable asynchronous trace writing
> 	perf record: extend trace writing to multi AIO

FYI I was rebasing my threads branch on top of this and
first 2 won't apply anymore on Arnaldo's perf/core

Arnaldo,
could we get this merged soon? the world around is moving
fast and we don't want 20th revision on this ;-)

thanks,
jirka