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

From: Alexey Budankov
Date: Tue Aug 28 2018 - 10:17:47 EST


On 28.08.2018 14:58, Alexey Budankov wrote:
> Hi Andi,
> On 28.08.2018 11:59, Jiri Olsa wrote:
>> 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?
> Is it ok to share VTune GUI screenshots I sent you the last time
> to demonstrate the advantage of AIO trace streaming?

VTune release manager permitted to share it, well, sorry for bothering.

> Thanks,
> Alexey
>> thanks,
>> jirka