Poor performance of sendfile in high load big file download

[arnoldguo]

My server is using to download big file(10M~20M each),
when using sendfile on ,if the conn reach 800-1.2k conn/s(limit_rate 150k),
the perfmance get poor,time_starttransfer may be 20~30s.
When I change to sendfile off,output_buffer  1 512k,when  conn reach 800-1.2k conn/s,
the performance is better,time_starttransfer<10s,but  the NIC IO is limit to 70-80MB/s.
My Env:
Xeon X 2(4 processce X 2),12G RAM,5 SATA as raid0
OS:AS5,Ngnix 0.8.15,
worker_processes 8;

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,13532,13532#msg-13532