1.9 - autoload_data.php - permission denied

i am continuing to setup 1.9rc1 here. now i see:

fopen(/mypath/system_cache/autoload_data.php): failed to open stream: Permission denied"

in the logs..

what is the correct permission level for this file? currently it is 644 and owned by the same user as i use to run the main web processes.

thanks