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Date: Wed, 22 Apr 2015 17:45:25 -0700
From: Stephen Hemminger <stephen@networkplumber.org>
To: Adrian Popescu <adriannnpopescu@gmail.com>
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Cc: cake@lists.bufferbloat.net
Subject: Re: [Cake] Cake3 - source code and some questions
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On Thu, 23 Apr 2015 00:02:35 +0300
Adrian Popescu <adriannnpopescu@gmail.com> wrote:

> e1000e (82574, dual port and quad port server e1000e adapters with
> various Intel chips) is exhibiting varying latency based on the used
> kernel.
> 
> Local network ping latencies on Ubuntu 14.04 with its LTS 3.13 kernel
> are always below 0.5 milliseconds when idle and lightly loaded. The
> latest kernels from kernel.org are unable to match those latencies.
> These modern kernels are always seeing latencies of 2-3 milliseconds.
> 
> Turning off all offloads on the involved ethernet e1000e network
> interfaces doesn't help. This is all physical Intel hardware with
> e1000e interfaces.
> 
> Two idle hosts on a local network should be able to ping each other
> and get latencies which are always less than one millisecond. FreeBSD
> doesn't have this problem.

These NIC's have had a histrory of power management related issues.
I suspect some power management (maybe even in SMI) is turning off
parts of the chips and it is taking long to turn back on.