[GN] Configure a pool for copy_bundle_data and compile_xcassets tools.

build/toolchain/mac/BUILD.gn

Index: build/toolchain/mac/BUILD.gn
diff --git a/build/toolchain/mac/BUILD.gn b/build/toolchain/mac/BUILD.gn
index 1fdc1eef3175805485521607d667a19981de86e3..eb3dc1881ac49bf879ce01f16c970da11e329236 100644
--- a/build/toolchain/mac/BUILD.gn
+++ b/build/toolchain/mac/BUILD.gn
@@ -23,6 +23,28 @@ if (use_goma) {
   goma_prefix = ""
 }
 
+declare_args() {
+  # Control the number of copy_bundle_data and compile_xcassets tasks to run
+  # simultaneously. As the tasks are doing lots of I/O, some limit is required
+  # to prevent I/O starvation when ninja is run with a large number of parallel
+  # jobs (e.g. when using distributed build like goma).
+  bundle_pool_depth = -1
+}
+
+if (current_toolchain == default_toolchain) {
+  # Reduce the number of tasks using the copy_bundle_data and compile_xcassets
+  # tools as they can cause lots of I/O contention when invoking ninja with a
+  # large number of parallel jobs (e.g. when using distributed build like goma).
+  pool("bundle_pool") {
+    if (bundle_pool_depth == -1) {

[Dirk Pranke, 2016/06/07 22:17:46]

Can we just use the value of concurrent_links here? It seems like adding another
build arg for this is overkill. 

It also seems like we can just re-use the existing (concurrent_links) value
rather than not having to call out to the script again. I suppose it's
theoretically possible that someone might set concurrent_links=X explicitly and
we want bundle_pool_depth=Y, but that seems like excessive optimization.

[sdefresne, 2016/06/08 08:04:27]

Done.

+      depth =
+          exec_script("//build/toolchain/get_concurrent_links.py", [], "value")
+    } else {
+      depth = bundle_pool_depth
+    }
+  }
+}
+
 # This will copy the gyp-mac-tool to the build directory. We pass in the source
 # file of the mac tool.
 gyp_mac_tool_source =
@@ -271,6 +293,7 @@ template("mac_toolchain") {
                           root_build_dir)
       command = "python $_tool {{source}} {{output}}"
       description = "COPY_BUNDLE_DATA {{source}} {{output}}"
+      pool = ":bundle_pool($default_toolchain)"
     }
     tool("compile_xcassets") {
       _tool = rebase_path("//build/toolchain/mac/compile_xcassets.py",
@@ -287,6 +310,7 @@ template("mac_toolchain") {
                 "-o {{output}} {{inputs}}"
 
       description = "COMPILE_XCASSETS {{output}}"
+      pool = ":bundle_pool($default_toolchain)"
     }
 
     toolchain_args() {