Re: [BUG] perf report: ordered events and flushing bug

David Ahern
2015-03-12T19:34:35Z

On 3/12/15 1:23 PM, Stephane Eranian wrote:
>> Rounds and flushing after them helps with the user experience -- at least
>> for some commands. On systems with 1024 cpus perf data files get large
>> quickly and the resulting analysis command can appear to hang for long
>> periods (e.g., i have done 1 second data collections that generate 5GB files
>> and take 30-45 minutes before perf shows any output).
>>
>> I have a patch to perf-record to allow a user to specify the poll timeout.
>> That helps define the maximum time between 'rounds' on the record side and
>> then commands that show each event (e.g, perf script) will show output much
>> more quickly without the need to consume large amounts of memory or process
>> the entire file before displaying events.
>>
> I can understand this but not at the cost of correctness. You have no guarantee
> that events will appear in the perf.data with a fixed maximum time window, i.e.,
> beyond the window, out-of-order events may be discarded.
> There ought to be a better way of keeping the user aware of what's going on
> with the progress bar.

Theoretically there are no events coming in out of order. Have you seen
the comment before process_finished_round() in util/session.c?

In practice it did happen occasionally and perf would dump:
Warning: Timestamp below last timeslice flush

and then terminate. That was removed recently by Jiri I think and events
out of the window are discarded (??).

In my case I don't want a progress bar, I want output and I don't need
the entire file processed for output to start. For a while I was
dropping back to the ftrace interface in debugfs b/c it was faster.

For the JIT case you tried appending events at the end of the file but
that makes them no where near the real time points so what you really
need is to insert events into the stream. Perhaps the separate files
option would help -- jit events in a separate file and inserted based on
time as the files are processed?

David