packet delay because of blackhole

[Rui Paulo]

On Fri, Mar 28, 2008 at 08:14:58PM +0300, Anthony Pankov wrote:
> Just for somebody convince.
> 
> While analyzing client<->server HTTPS conversation one second delay in
> packet exchange was discovered (strongly reproducible):
> 
> Sample:
> N        time
> 6       0.002303        10.28.4.14      10.28.4.50      SSL     Client Hello
> 7       0.106710        10.28.4.50      10.28.4.14      TCP     443 > 1447 [ACK] Seq=1 Ack=103 Win=65535 Len=0
> 8       1.045712        10.28.4.50      10.28.4.14      TLSv1   Server Hello, Certificate, Server Hello Done
> 
> Another sample:
> 10      0.011722        10.28.4.14      10.28.4.50      TLSv1   Application Data
> 11      0.115933        10.28.4.50      10.28.4.14      TCP     443 > 1442 [ACK] Seq=839 Ack=519 Win=65466 Len=0
> 12      1.054037        10.28.4.50      10.28.4.14      TLSv1   Application Data
> 
> The reason for delay is sysctl tcp.blackhole value grater than 0, much to surprise.
> 
> So, turning tcp.blackhole to 0 eliminate any delay (strongly reproducible).
> 
> System: FreeBSD 6_2_stable


I'm not sure how performance penalty can induce a cache miss and I
it's very processor specific. So, you're best guess is to profile the
kernel.

Regards,
-- 
Rui Paulo