Android source build problem on Virtual machine

You may install Ubuntu on Virtualbox, and use it to build Android source. When you build whole source code based on Android 1.6, you will not meet "too many open file" error when trying to build it. But when you run "repo sync" to update Android 1.6 to 2.1, you may meet this error.
This is caused by the small open file limitation on Ubuntu Linux system.
You can add following value into configuration file in /etc/security/limits.conf


jacky soft nofile 2048
jacky hard nofile 4096

This changing will let the user(jacky) can open files up to 4096.

Then the "too many open file" error will go out.