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
