[PATCH v2 0/5] Benchmark and improve event synthesis performance

Ian Rogers
2020-04-02T15:44:05Z

Event synthesis is performance critical in common tasks using perf. For
example, when perf record starts in system wide mode the /proc file
system is scanned with events synthesized for each process and all
executable mmaps. With large machines and lots of processes, we have seen
O(seconds) of wall clock time while synthesis is occurring.

This patch set adds a benchmark for synthesis performance in a new
benchmark collection called 'internals'. The benchmark uses the
machine__synthesize_threads function, single threaded on the perf process
with a 'tool' that just drops the events, to measure how long synthesis
takes.

By profiling this benchmark 2 performance bottlenecks were identified,
hugetlbfs_mountpoint and stdio. The impact of theses changes are:

Before:
Average synthesis took: 167.616800 usec
Average data synthesis took: 208.655600 usec

After hugetlbfs_mountpoint scalability fix:
Average synthesis took: 120.195100 usec
Average data synthesis took: 156.582300 usec

After removal of stdio in /proc/pid/maps code:
Average synthesis took: 67.189100 usec
Average data synthesis took: 102.451600 usec

Time was measured on an Intel Xeon 6154 compiling with Debian gcc 9.2.1.

v2 of this patch set adds the new benchmark to the perf-bench man page
and addresses review comments from Jiri Olsa, thanks!

Two patches in the set were sent to LKML previously but are included
here for context around the benchmark performance impact:
https://lore.kernel.org/lkml/20200327172914.28603-1-irogers@google.com/T/#u
https://lore.kernel.org/lkml/20200328014221.168130-1-irogers@google.com/T/#u

A future area of improvement could be to add the perf top
num-thread-synthesize option more widely to other perf commands, and
also to benchmark its effectiveness.

Ian Rogers (4):
perf bench: add event synthesis benchmark
perf synthetic-events: save 4kb from 2 stack frames
tools api: add a lightweight buffered reading api
perf synthetic events: Remove use of sscanf from /proc reading

Stephane Eranian (1):
tools api fs: make xxx__mountpoint() more scalable

tools/lib/api/fs/fs.c | 17 +++
tools/lib/api/fs/fs.h | 12 ++
tools/lib/api/io.h | 107 ++++++++++++++
tools/perf/Documentation/perf-bench.txt | 8 ++
tools/perf/bench/Build | 2 +-
tools/perf/bench/bench.h | 2 +-
tools/perf/bench/synthesize.c | 101 ++++++++++++++
tools/perf/builtin-bench.c | 6 +
tools/perf/util/synthetic-events.c | 177 +++++++++++++++---------
9 files changed, 367 insertions(+), 65 deletions(-)
create mode 100644 tools/lib/api/io.h
create mode 100644 tools/perf/bench/synthesize.c

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2.26.0.rc2.310.g2932bb562d-goog