Re: [PATCH RFC V4 5/6] perf top: switch to backward overwrite mode

[Jiri Olsa]

On Fri, Sep 29, 2017 at 07:47:56AM -0700, kan.liang@xxxxxxxxx wrote:
> From: Kan Liang <kan.liang@xxxxxxxxx>
> 
> perf_top__mmap_read has severe performance issue in
> Knights Landing/Mill, when monitoring in heavy load system. It costs
> several minutes to finish, which is unacceptable.
> 
> perf top was overwrite mode. But it is changed to non overwrite mode
> since commit 93fc64f14472 ("perf top: Switch to non overwrite mode").
> For non overwrite mode, it tries to read everything in the ring buffer
> and does not check the messup. Once there are lots of samples delivered
> shortly, the processing time could be very long.
> Knights Landing/Mill as a manycore processor contains a large number of
> small cores. Because of the huge core number, it will generated lots of
> samples in a heavy load system. Also, since the huge sample#, the mmap
> writer probably bite the tail and mess up the samples.
> 
> Also, to avoid the problems which is described in 
> commit 9ecda41acb97 ("perf/core: Add ::write_backward attribute to perf
> event"), switch to backward overwrite mode.
> Pausing the ring-buffer during perf_top__mmap_read to ensure the
> ring-buffer is stable.
> There would be some records lost in backward overwrite mode. Removing
> the lost events checking.

I'm getting perf top hogging the cpu completely with this change

jirka