Re: DCD and TCP timeout

From: Riyaj Shamsudeen <riyaj.shamsudeen_at_gmail.com>
Date: Tue, 12 Nov 2013 15:35:32 -0800
Message-ID: <CAA2DszyVqWAY1rkv4q=Jj=6nPQ8bOc035DA9Zmk=MoxMu=4m=A_at_mail.gmail.com>

Amir
  Setting expire_time to 1, will send a SQLNET packet every minute ( so a TCP/IP probe is sent every minute). In normal conditions, a TCP ACK will be received immediately.

 But, if the client forcefully dies or kills the connections, then TCP retransmission code kicks-in for the unacknowledged TCP/IP transmission. The tcp_retries2 kernel parameter controls the behavior of retransmission, in LINUX. In a connection ESTABLISED state, TCP/IP retransmits 15 times (default value of tcp_retries2 kernel parameter) , with an exponential backoff for TCP retransmission interval, before raising an alarm to the application. Read the link below, and I think, I had similar results as your test case, last time I performed the test. This behavior is only applicable to LINUX.

  Do you really care to change the TCP level parameters? If yes, you can reduce the tcp_retries2 parameter to a value >8 and test it.( Please let me know if you still see a different behavior after the adjustment). If that isn't enough, tcp_keepalive_time can be reduced to 10 minutes, but it can increase network traffic marginally (one tcp keep alive parameter every 10 minutes to all alive TCP/IP sockets).

 Read :
http://stackoverflow.com/questions/5907527/application-control-of-tcp-retransmission-on-linux

 HTH Cheers

Riyaj Shamsudeen
Principal DBA,
Ora!nternals - http://www.orainternals.com - Specialists in Performance, RAC and EBS
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Received on Wed Nov 13 2013 - 00:35:32 CET