Messages in this thread | | | Subject | Re: [PATCH v8 0/3]: perf: reduce data loss when profiling highly parallel CPU bound workloads | From | Alexey Budankov <> | Date | Mon, 10 Sep 2018 13:40:17 +0300 |
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Hi Ingo,
On 10.09.2018 12:18, Ingo Molnar wrote: > > * Alexey Budankov <alexey.budankov@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 >
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