* Alexey Budankov <[email protected]> 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). 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? Thanks, Ingo
