* File Formats

   - gfmd-thread-dependency.dot is Graphviz fromat.

   - gfmd-thread-dependency.png is made by dot command.
     To make it, execute 'make' command.

   - gfarm-v2-schema*.svgz are gzipped inkscape svg format.

   - gfarm-v2-schema*.pdf are saved as PDF format by inkscape.

* Rules for gfmd-thread-dependency

   - color of arrows 

     green    : network
     sky blue : db
     blue     : thread pool
     red      : mutex

