Messages in this thread | | | Date | Wed, 27 Sep 2017 14:00:21 +0200 | From | Jiri Olsa <> | Subject | Re: [PATCH RFC V3 5/5] perf top: switch back to overwrite mode |
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On Mon, Sep 25, 2017 at 01:23:08PM -0700, kan.liang@intel.com wrote: > From: Kan Liang <kan.liang@intel.com> > > 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. > > Switching to overwrite mode, which dropping the unsure mmap entries, > significantly speeds up the whole progress. > Considering the real time requirement for perf top, it should switch > back to overwrite mode. > > Only warning once if the messup is detected. > Providing some hints to users. > > Signed-off-by: Kan Liang <kan.liang@intel.com> > --- > tools/perf/builtin-top.c | 2 +- > tools/perf/util/evlist.c | 5 ++++- > 2 files changed, 5 insertions(+), 2 deletions(-) > > diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c > index 477a869..4b4af34 100644 > --- a/tools/perf/builtin-top.c > +++ b/tools/perf/builtin-top.c > @@ -902,7 +902,7 @@ static int perf_top__start_counters(struct perf_top *top) > } > } > > - if (perf_evlist__mmap(evlist, opts->mmap_pages, false) < 0) { > + if (perf_evlist__mmap(evlist, opts->mmap_pages, true) < 0) {
hum, I thought that it's not as simple as using 'true' in here, because of the issue explained in here: 9ecda41acb97 perf/core: Add ::write_backward attribute to perf event
jirka
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