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SubjectRe: [PATCH v8 0/3]: perf: reduce data loss when profiling highly parallel CPU bound workloads

* 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).

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

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