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

[Jiri Olsa]

On Mon, Sep 10, 2018 at 11:18:41AM +0200, Ingo Molnar wrote:
> 
> * Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx> 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).

this patch adds the aoi for writing to the perf.data
file, reading of events is unchanged

> 
> 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?

it's still usable, I can rebase it and post a branch pointer,
the problem is I haven't been able to find a case with a real
performance benefit yet.. ;-)

perhaps because I haven't tried on server with really big cpu
numbers

jirka