Add doc about how to benchmark OpenJDK
Change-Id: I493fc77f2e9646e0c09c984ab74c35b7ff0b8257
diff --git a/build_performance.md b/build_performance.md
new file mode 100644
index 0000000..7847a24
--- /dev/null
+++ b/build_performance.md
@@ -0,0 +1,72 @@
+# Benchmarking Build Performance of OpenJDK
+
+Documentation:
+[Updating OpenJDK for Android Platform Builds](http://go/updating-opendjk-android)
+
+### 1. Build openjdk17 locally
+
+```
+PATH/TO/openjdk/repo/toolchain/jdk/build/build-openjdk17-linux.sh -d destdir builddir
+```
+
+ The build writes `jdk.zip` and `jdk-debuginfo.zip` to `destdir`.
+
+### 2. Updating the platform prebuilts from a local build
+
+```
+rm -rf PATH/TO/internal/repo/main/prebuilts/jdk/jdk17/linux-x86
+mkdir PATH/TO/internal/repo/main/prebuilts/jdk/jdk17/linux-x86
+unzip $DIST_DIR/jdk.zip -d PATH/TO/internal/repo/main/prebuilts/jdk/jdk17/linux-x86
+```
+ Use `jdk.zip` to populate `PATH/TO/internal/repo/main/prebuilts/jdk/jdk17/linux-x86`.
+
+### 3. Disable hyperthreading on gLinux server
+
+ Public Documents: [How to disable or enable hyper threading on my Linux server](https://www.golinuxhub.com/2018/05/how-to-disable-or-enable-hyper/)
+
+ 1. Check whether hyperthreading is enabled and show all of the logical CPUs
+ and their HT pair relationships.
+
+ ```
+ grep -H . /sys/devices/system/cpu/cpu*/topology/thread_siblings_list | sort -n -t ',' -k 2 -u
+ ```
+
+ 2. If hyperthreading is enabled, disable it. (assume there are n CPUs)
+
+ Create a script to disable hyperthreading.
+
+ ```
+ $ cat > /tmp/disable_ht.sh <<EOF
+ #!/bin/bash
+ for i in {0..n/2}; do
+ echo "Disabling logical HT core $i."
+ echo 0 > /sys/devices/system/cpu/cpu${i}/online;
+ done
+ EOF
+ sh /tmp/disable_ht.sh
+ ```
+
+### 4. Do the clean build of internal android
+
+ ```
+ rm -rf `PATH/TO/internal/repo/main/out`
+ USE_RBE=false m -j32
+ ```
+
+### 5. Analyze `PATH/TO/internal/repo/main/out/build.trace.gz`
+
+ ```
+ gunzip PATH/TO/internal/repo/main/out/build.trace.gz
+ ```
+
+ Then you will get a json file called `build.trace`. Collect all file whose `rule_name`
+ is `javac` and then the sum of their `user_time` is the total time.
+ Two methods are provided below. Both work.
+
+ #### 5.1 You can write a python script to analyze data
+
+ #### 5.2 command line version
+
+ ```
+ grep rule_name.*javac PATH/TO/build.trace | cut -d '"' -f 13 | cut -c2- | sed 's/.$//' | awk '{sum += $1} END {print sum}'
+ ```
