Build Performance

Debugging Build Performance


soong_ui has tracing built in, so that every build execution‘s trace can be viewed. Just open $OUT_DIR/build.trace.gz in Chrome’s chrome://tracing, or with catapult's trace viewer. The last few traces are stored in build.trace.#.gz (larger numbers are older). The associated logs are stored in soong.#.log.

trace example


Soong can be traced and profiled using the standard Go tools. It understands the -cpuprofile, -trace, and -memprofile command line arguments, but we don't currently have an easy way to enable them in the context of a full build.


In general, the slow path of reading files isn‘t particularly performance sensitive, since it doesn’t need to happen on every build. It is important for the fast-path (detecting whether it needs to regenerate the ninja file) to be fast however. And it shouldn‘t hit the slow path too often -- so don’t rely on output of a $(shell) command that includes the current timestamp, or read a file that's going to change on every build.

Regen check is slow

In most cases, we've found that the fast-path is slow because all of the $(shell) commands need to be re-executed to determine if their output changed. The $OUT_DIR/soong.log contains statistics from the regen check:

.../kati.go:127: *kati*: regen check time: 1.699207
.../kati.go:127: *kati*: glob time (regen): 0.377193 / 33609
.../kati.go:127: *kati*: shell time (regen): 1.313529 / 184
.../kati.go:127: *kati*:  0.217 find device vendor -type f -name \*.pk8 -o -name verifiedboot\* -o -name \*.x509.pem -o -name oem\*.prop | sort
.../kati.go:127: *kati*:  0.105 cd packages/apps/Dialer ; find -L . -type d -name "res"
.../kati.go:127: *kati*:  0.035 find device vendor -maxdepth 4 -name '*' -o -name '*' | sort
.../kati.go:127: *kati*:  0.029 cd frameworks/base ; find -L  core/java graphics/java location/java media/java media/mca/effect/java media/mca/filterfw/java media/mca/filterpacks/java drm/java opengl/java sax/java telecomm/java telephony/java wifi/java lowpan/java keystore/java rs/java ../opt/telephony/src/java/android/telephony ../opt/telephony/src/java/android/telephony/gsm ../opt/net/voip/src/java/android/net/rtp ../opt/net/voip/src/java/android/net/sip   -name "*.html" -and -not -name ".*"
.../kati.go:127: *kati*:  0.025 test -d device && find -L device -maxdepth 4 -path '*/marlin/'
.../kati.go:127: *kati*:  0.023 find packages/apps/Settings/tests/robotests -type f -name '*' | sed -e 's!.*\(com/google.*Test\)\.java!\1!' -e 's!.*\(com/android.*Test\)\.java!\1!' | sed 's!/!\.!g' | cat
.../kati.go:127: *kati*:  0.022 test -d vendor && find -L vendor -maxdepth 4 -path '*/marlin/'
.../kati.go:127: *kati*:  0.017 cd cts/tests/tests/shortcutmanager/packages/launchermanifest ; find -L  ../src -name "*.java" -and -not -name ".*"
.../kati.go:127: *kati*:  0.016 cd cts/tests/tests/shortcutmanager/packages/launchermanifest ; find -L  ../../common/src -name "*.java" -and -not -name ".*"
.../kati.go:127: *kati*:  0.015 cd libcore && (find luni/src/test/java -name "*.java" 2> /dev/null) | grep -v -f java_tests_blacklist
.../kati.go:127: *kati*: stat time (regen): 0.250384 / 4405

In this case, the total time spent checking was 1.69 seconds, even though the other “(regen)” numbers add up to more than that (some parts are parallelized where possible). The biggest contributor is the $(shell) times -- 184 executions took a total of 1.31 seconds. The top 10 longest shell functions are printed.

All the longest commands in this case are all variants of a call to find, but this is where using pure make functions instead of calling out to the shell can make a performance impact -- many calls to check if 26 > 20 can add up. We've added some basic math functions in to help some common use cases that used to be rather expensive when they were used too often.

There are some optimizations in place for find commands -- if Kati can understand the find command, the built-in find emulator can turn some of them into glob or stat checks (falling back to calling find if one of those imply that the output may change). Many of the common macros produce find commands that Kati can understand, but if you‘re writing your own, you may want to experiment with other options if they’re showing up in this list. For example, if this was significantly more expensive (either in runtime, or was called often):

.../kati.go:127: *kati*:  0.015 cd libcore && (find luni/src/test/java -name "*.java" 2> /dev/null) | grep -v -f java_tests_blacklist

It may be more efficient to move the grep into make, so that the find portion can be rewritten and cached:

$(filter-out $(file <$(LOCAL_PATH)/java_tests_blacklist),$(call all-java-files-under,luni/src/test/java))

Others can be simplified by just switching to an equivalent find command that Kati understands:

.../kati.go:127: *kati*:  0.217 find device vendor -type f -name \*.pk8 -o -name verifiedboot\* -o -name \*.x509.pem -o -name oem\*.prop | sort

By adding the implicit -a and moving the | sort to Make, this can now be cached by Kati:

$(sort $(shell find device vendor -type -f -a -name \*.pk8 -o -name verifiedboot\* -o -name \*.x509.pem -o -name oem\*.prop))

Kati is learning about the implicit -a in this change

Kati regens too often

Kati prints out what triggered the slow path to be taken -- this can be a changed file, a changed environment variable, or different output from a $(shell) command:

out/soong/ was modified, regenerating...

The state is stored in $OUT_DIR/.kati_stamp* files, and can be (partially) read with the ckati_stamp_dump tool in prebuilts/build-tools. More debugging is available when ckati is run with --regen_debug, but that can be a lot of data to understand.


Understanding why something rebuilt

Add NINJA_ARGS="-d explain" to your environment before a build, this will cause ninja to print out explanations on why actions were taken. Start reading from the beginning, as this much data can be hard to read:

$ cd art
$ mma
$ touch runtime/jit/profile_compilation_info.h
$ NINJA_ARGS="-d explain" mma
ninja explain: output out/soong/.intermediates/art/tools/cpp-define-generator/cpp-define-generator-data/linux_glibc_x86_64/obj/art/tools/cpp-define-generator/main.o older than most recent input art/runtime/jit/profile_compilation_info.h (
1516683538 vs 1516685188)
ninja explain: out/soong/.intermediates/art/tools/cpp-define-generator/cpp-define-generator-data/linux_glibc_x86_64/obj/art/tools/cpp-define-generator/main.o is dirty
ninja explain: out/soong/.intermediates/art/tools/cpp-define-generator/cpp-define-generator-data/linux_glibc_x86_64/cpp-define-generator-data is dirty
ninja explain: out/soong/host/linux-x86/bin/cpp-define-generator-data is dirty
ninja explain: out/soong/.intermediates/art/tools/cpp-define-generator/cpp-define-generator-asm-support/gen/asm_support_gen.h is dirty
ninja explain: out/soong/.intermediates/art/cmdline/art_cmdline_tests/android_arm_armv7-a_core_cmdline_parser_test/obj/art/cmdline/cmdline_parser_test.o is dirty

In this case, art/cmdline/cmdline_parser_test.o was rebuilt because it uses asm_support_gen.h, which was generated by cpp-define-generator-data, which uses profile_compilation_info.h.

You'll likely need to cross-reference this data against the build graph in the various .ninja files. The files are (mostly) human-readable, but a (slow) web interface can be used by running NINJA_ARGS="-t browse <target>" m.

Builds take a long time

If the long part in the trace view of a build is a relatively solid block, then the performance is probably more related to how much time the actual build commands are taking than having extra dependencies, or slowdowns in soong/kati/ninja themselves.

Beyond looking at visible outliers in the trace view, we don‘t have any tooling to help in this area yet. It’s possible to aggregate some of the raw data together, but since our builds are heavily parallelized, it‘s particularly easy for build commands to impact unrelated build commands. This is an area we’d like to improve -- we expect keeping track of user/system time per-action would provide more reliable data, but tracking some full-system data (memory/swap use, disk bandwidth, etc) may also be necessary.

Known Issues



Soong always loads the entire module graph, so as modules convert from Make to Soong, mm is becoming closer to mma. This produces more correct builds, but does slow down builds, as we need to verify/produce/load a larger build graph.

We‘re exploring a few options to speed up build startup, one being an experimental set of ninja patches, though that’s not the current path we're working towards.

Android 8.1 (Oreo MR1)

In some cases, a tree would get into a state where Soong would be run twice on every incremental build, even if there was nothing to do. This was fixed in master with these changes, but they were too significant to backport at the time. And while they fix this particular issue, they appear to cause ninja to spend more time during every build loading the .ninja_log / .ninja_deps files, especially as they become larger.

A workaround to get out of this state is to remove the entry from $OUT_DIR/.ninja_log:

sed -i "/\/" $(get_build_var OUT_DIR)/.ninja_log