Run benchmarks serially.

This prevent a benchmark from running while another benchmark was running or code is compiling, improving the quality of the benchmark.

Change-Id: I4f521ae70b66eb813dfbdf4cce10d3ee6a6a6964
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/25761
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Commit-Queue: Matt Stark <msta@google.com>

tools/jj/config.toml

diff --git a/tools/jj/config.toml b/tools/jj/config.toml
index 9546641..5394e60 100644
--- a/tools/jj/config.toml
+++ b/tools/jj/config.toml
@@ -18,13 +18,26 @@
 patterns = ["null"]
 
 [aliases]
-compare = [
+compare-internal = [
   "run",
-  "-j2",
   "--revision=before=before",
   "--revision=after=@",
   # Allow running on immutable commits
   "--ignore-changes",
+]
+
+compare = [
+  "compare-internal",
+  "-j2",
+  "--",
+  "bash",
+  "-c",
+]
+
+compare-serial = [
+  "compare-internal",
+  "-j1",
+  "--passthrough",
   "--",
   "bash",
   "-c",
@@ -36,6 +49,11 @@
   "python3 build/gen.py --out-path=/tmp/$JJ_LABEL ${GN_OPTS:-} && ninja -C /tmp/$JJ_LABEL ${NINJA_TARGETS:-gn}",
 ]
 
+before-after-serial = [
+  "compare-serial",
+  "python3 build/gen.py --out-path=/tmp/$JJ_LABEL ${GN_OPTS:-} && ninja -C /tmp/$JJ_LABEL ${NINJA_TARGETS:-gn}",
+]
+
 compare-perf = [
   "util",
   "exec",
@@ -43,9 +61,10 @@
   "bash",
   "-c",
   """
+  # We need to use serial because we can't run two performance benchmarks at the same time.
   GN_OPTS="--gen=${SRC:-$HOME/chromium/src}=${OUT_DIR:-out/\\$JJ_LABEL}" \
   NINJA_TARGETS=gen_bench \
-  jj --config "revset-aliases.before=\\"${BEFORE:-trunk()}\\"" before-after && \
+  jj --config "revset-aliases.before=\\"${BEFORE:-trunk()}\\"" before-after-serial && \
   python3 "$JJ_WORKSPACE_ROOT/tools/compare_traces.py" \
     --before /tmp/before/gen_bench_[0-9]*.trace \
     --after  /tmp/after/gen_bench_[0-9]*.trace