How can AOSP git repository be so fast to clone?

[程洋]

My original post is on stackoverflow

There are some more info here:

1. i now know using "gerrit gc" will build bitmap, which drastically decrase the time cost of "couting objects" phase

2. However, other 2 phases "cloning into" and "finding sources" are still slow than google

3. "cloning into" phase cost me 24 seconds and google take less than 1 second.

4. "finding sources" phase cost me 2 minute and google take less than 1 second

5. it seems "finding sources" on google says only 153/153, and my project says its 4231671/4231671

Is there any clue? especially the "finding sources". i wish someone can help me to understand it