TCP RTO During Overload Conditions

Onload User Guide (UG1586)

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1.2 English

Using Onload, under very high load conditions an increased frequency of TCP retransmission timeouts (RTOs) might be observed. This has the potential to occur when a thread servicing the stack is descheduled by the CPU whilst still holding the stack lock thus preventing another thread from accessing/polling the stack. A stack not being serviced means that ACKs are not received in a timely manner for packets sent, resulting in RTOs for the unacknowledged packets and increased jitter on the Onload stack.

Enabling the per stack environment variable EF_INT_DRIVEN can reduce the likelihood of this behavior and reduce jitter by ensuring the stack is serviced promptly. TCP with Jumbo Frames

When using jumbo frames with TCP, Onload will limit the MSS to 2048 bytes to ensure that segments do not exceed the size of internal packet buffers.

This should present no problems unless the remote end of a connection is unable to negotiate this lower MSS value.