what's wrong with graphviz in php-fpm mode?

I try to config a program use xhprof.

When I use php-cli mode or php's built in webServer, the callgraph generate image works well.

But When I use nginx+php-fpm, the dot exec at xhprof_generate_image_by_dot blocks forever.

Then I install the pear Image_Graphviz.Write a simple case like this:

require_once  'Image/GraphViz.php';
$img = new Image_GraphViz();
$img->addNode(
    'Node1',
    array(
        'URL'   => 'http://link1',
        'label' => 'This is a label',
        'shape' => 'box'
    )
);

$img->image('png');

php's built in webServer can generate png files ok,but php-fpm block at dot exec forever.

So, can anybody help me? What's wrong with this? Here is some relevant machine information:

• The OS: OSX
• The graphviz version:2.34.0
• The App: nginx1.2.8+php-fpm+php5.4.21+xhprof(the latest version from github)

After reboot my machine.Everything turn to right.

Mystery.