Hi Ingo, On 10.09.2018 12:18, Ingo Molnar wrote: > > * Alexey Budankov <alexey.budan...@linux.intel.com> 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. > > Yay! I saw this frequently on a 120-CPU box (hw is broken now). > >> 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%. > > Hm, instead of AIO why don't we use explicit threads instead? I think Posix > AIO will fall back > to threads anyway when there's no kernel AIO support (which there probably > isn't for perf > events).
Explicit threading is surely an option but having more threads in the tool that stream performance data is a considerable design complication. Luckily, glibc AIO implementation is already based on pthreads, but having a writing thread for every distinct fd only. > > Per-CPU threading the record session would have so many other advantages as > well (scalability, > etc.).> > Jiri did per-CPU recording patches a couple of months ago, not sure how > usable they are at the > moment? Tool threads may contend, and actually do, with application threads, under heavy load when all CPU cores are utilized, and this may alter performance profile. Thanks, Alexey > > Thanks, > > Ingo >
