netdev - Cause of Large Latency Difference Between 1179-byte and 1180-byte UDP Frames?

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Message-ID: <CAE1W6VpPJwKtFGmeLuStt-A8JjGj2xU8RitonBHBs3dH9pXYRA@mail.gmail.com>
Date:	Wed, 20 May 2015 21:50:04 -0700
From:	Todd Bezenek <bezenek@...il.com>
To:	netdev@...r.kernel.org
Subject: Cause of Large Latency Difference Between 1179-byte and 1180-byte UDP Frames?

I'm pushing 10,000 frames per second of UDP traffic through a Linux
system with a bridge configured between two 1GbE ports.

Iptables is installed and running, but the default rule is ACCEPT with
no other rules.

When I make the packets 1179 bytes in size (total size includes
Ethernet header, etc.), I see the latency of most packets between 60
and 200 usec.  When I change the size to 1180 bytes, I start seeing
about 1% of the packets with latencies larger than 300 usec, and some
as high as 500+ usec.

Any ideas what buffer size, cache line count, TLB reach, etc. might be
causing this dramatic change?

Cut-and-paste to my terminal is not working, so limited info here:

Kernel:  3.12.20 x86-64
Processor:  Intel E3845 (Atom, 4 cores)
Network interface driver:  Intel igb

Thank you for any helpful pointers.

-Todd

-- 
Todd Bezenek
bezenek@...il.com

A people hire A people, B people hire C people.
  --Jim Gray
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