prog('bash', '-c',...) will block
i spawn 10 prog to execute a curl command use ngx.thread.spawn. i had tried follow two way:

 for i=1, 10 do prog('curl', '-L', '-s', 'www.xxx.com' ) end 

 for i=1, 10 do prog('bash', '-c', 'curl -L -s www.xxx.com' ) end 

it spend seconds mostly same as one curl when use first way. it work as my expected. but when i use the second way, it seems execute one by one. it spend 10 times senconds. why this, 3q