gcc has options to instrument the generated executable and use the
collected information to better optimize the generated code.
To use the profile feedback options, you have to compile twice. After
the first compilation, you have to run the executable with a
representative set of data to generate the statistics for the second
compilation.
A benchmark was chosen as the representative set of data because a
benchmark focuses on measuring the code speed. A good benchmark should
be designed considering how git is commonly used, emphasizing the most
used parts, but without neglecting the less used parts.
To simplify the patch and focus on the Makefile rules, a dummy benchmark
was used, which does only a few simple operations a hundred times.
Even with just that simple benchmark, it seems to already have some
effect. These were the results of a test run of "./git fsck --full" on
the git repository, recorded via perf stat, ran twice without profile
feedback and then twice with profile feedback:
Without profile feedback:
Performance counter stats for './git fsck --full':
84159,595732 task-clock-msecs # 0,998 CPUs
8470 context-switches # 0,000 M/sec
7 CPU-migrations # 0,000 M/sec
263682 page-faults # 0,003 M/sec
179028259343 cycles # 2127,247 M/sec
222624952497 instructions # 1,244 IPC
22651880335 branches # 269,154 M/sec
1798298585 branch-misses # 7,939 %
1380706193 cache-references # 16,406 M/sec
588704688 cache-misses # 6,995 M/sec
84,360035244 seconds time elapsed
Performance counter stats for './git fsck --full':
84207,836012 task-clock-msecs # 0,998 CPUs
8450 context-switches # 0,000 M/sec
2 CPU-migrations # 0,000 M/sec
211878 page-faults # 0,003 M/sec
179131407549 cycles # 2127,253 M/sec
222395625483 instructions # 1,242 IPC
22614432765 branches # 268,555 M/sec
1798786420 branch-misses # 7,954 %
1384878082 cache-references # 16,446 M/sec
594115144 cache-misses # 7,055 M/sec
84,404733868 seconds time elapsed
With profile feedback:
Performance counter stats for './git fsck --full':
83576,534090 task-clock-msecs # 0,998 CPUs
8610 context-switches # 0,000 M/sec
10 CPU-migrations # 0,000 M/sec
211882 page-faults # 0,003 M/sec
177777232502 cycles # 2127,119 M/sec
200278330301 instructions # 1,127 IPC
21014495344 branches # 251,440 M/sec
1659583500 branch-misses # 7,897 %
2297603485 cache-references # 27,491 M/sec
1314536406 cache-misses # 15,729 M/sec
83,783009768 seconds time elapsed
Performance counter stats for './git fsck --full':
83288,834113 task-clock-msecs # 0,998 CPUs
8365 context-switches # 0,000 M/sec
5 CPU-migrations # 0,000 M/sec
174403 page-faults # 0,002 M/sec
177173914267 cycles # 2127,223 M/sec
200234762879 instructions # 1,130 IPC
21006452953 branches # 252,212 M/sec
1659600980 branch-misses # 7,900 %
2298086212 cache-references # 27,592 M/sec
1309056989 cache-misses # 15,717 M/sec
83,482945132 seconds time elapsed
Signed-off-by: Cesar Eduardo Barros <cesarb@xxxxxxxxxx>
---
Makefile | 20 ++++++++++++++++++++
bench.sh | 45 +++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 65 insertions(+), 0 deletions(-)
create mode 100755 bench.sh
diff --git a/Makefile b/Makefile
index e40ac0c..8eaa8a0 100644
--- a/Makefile
+++ b/Makefile
@@ -2486,3 +2486,23 @@ cover_db: coverage-report
cover_db_html: cover_db
cover -report html -outputdir cover_db_html cover_db
+
+### Profile feedback based optimization
+#
+.PHONY: benchmark profile-generate profile-use profile-optimized-build
+
+benchmark:
+ time ./bench.sh
+
+profile-generate:
+ $(MAKE) clean coverage-clean
+ $(MAKE) CFLAGS="$(CFLAGS) -fprofile-generate" LDFLAGS="$(LDFLAGS) -fprofile-generate" all
+
+profile-use:
+ $(MAKE) clean
+ $(MAKE) CFLAGS="$(CFLAGS) -fprofile-use" LDFLAGS="$(LDFLAGS) -fprofile-use" all
+
+profile-optimized-build:
+ $(MAKE) profile-generate
+ $(MAKE) -j1 benchmark
+ $(MAKE) profile-use
diff --git a/bench.sh b/bench.sh
new file mode 100755
index 0000000..939f3b3
--- /dev/null
+++ b/bench.sh
@@ -0,0 +1,45 @@
+#!/bin/bash
+set -eu
+
+BENCHDIR="$PWD/bench"
+GIT_BUILD_DIR="$BENCHDIR/.."
+export PATH="$GIT_BUILD_DIR/bin-wrappers:$PATH"
+
+mkdir "$BENCHDIR"
+cd "$BENCHDIR"
+
+git --version
+exec > bench.log 2>&1
+
+for count in `seq 100`; do
+
+mkdir "$BENCHDIR/test"
+cd "$BENCHDIR/test"
+git init
+echo a > a
+echo b > b
+git add a b
+git commit -m 'Commit 1'
+echo c > c
+echo d > d
+git add c d
+git commit -m 'Commit 2'
+git checkout -b branch
+echo e > e
+echo f > f
+git add e f
+git commit -m 'Commit 3'
+git checkout -
+echo g > g
+echo h > h
+git add g h
+git commit -m 'Commit 4'
+git merge branch
+git fsck --full
+cd "$BENCHDIR"
+rm -rf test
+
+done
+
+cd ..
+rm -rf "$BENCHDIR"
--
1.7.4.4
--
To unsubscribe from this list: send the line "unsubscribe git" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html