[Docs] Update the Mac and iOS docs for `git status` perf improvements.
BUG=none
TEST=none
R=sdy@chromium.org
Review-Url: https://codereview.chromium.org/2876983004
Cr-Commit-Position: refs/heads/master@{#471477}
diff --git a/docs/ios/build_instructions.md b/docs/ios/build_instructions.md
index 9aa2539..b841bee 100644
--- a/docs/ios/build_instructions.md
+++ b/docs/ios/build_instructions.md
@@ -260,6 +260,8 @@
### Improving performance of `git status`
+#### Increase the vnode cache size
+
`git status` is used frequently to determine the status of your checkout. Due
to the large number of files in Chromium's checkout, `git status` performance
can be quite variable. Increasing the system's vnode cache appears to help.
@@ -286,8 +288,22 @@
Or edit the file directly.
-If `git --version` reports 2.6 or higher, the following may also improve
-performance of `git status`:
+#### Configure git to use an untracked cache
+
+If `git --version` reports 2.8 or higher, try running
+
+```shell
+$ git update-index --test-untracked-cache
+```
+
+If the output ends with `OK`, then the following may also improve performance of
+`git status`:
+
+```shell
+$ git config core.untrackedCache true
+```
+
+If `git --version` reports 2.6 or higher, but below 2.8, you can instead run
```shell
$ git update-index --untracked-cache
