netdev - cpu usage on blocking network call

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Message-ID: <CA+q+rD2O8dkBhgxCsHVsq=j7zi56nb9D59gYHHA=uOKbnZZdSQ@mail.gmail.com>
Date:	Tue, 31 Jul 2012 14:14:01 -0500
From:	Robert Engels <rengels@...ionscity.com>
To:	netdev@...r.kernel.org
Subject: cpu usage on blocking network call

I have a strange problem, and was hoping the developers could provide
some insight.

I have tested with ubuntu 12.04, with the 3.2.0-23 lowlatency kernel.
I also tested with a 3.0.31 vanilla realtime kernel on a different
machine.

I open a blocking socket (and connect to a remote client), and start
two threads, a reader and a writer.

I pump LOTS of data down the writer, but do not receive any data on
the reader (by design).

The reader thread is reporting some CPU usage (not a lot, about 10% of
the writer cpu). I determine this using "top", and other profiling
tools.

I also run strace on the thread, and it shows that it is blocked in
read() call, and that is doesn't return.

Why/How is the reader thread consuming CPU? Poorly written driver?
Kernel bug? Or is this expected behavior or the kernel?

I am more than willing to spend more time tracking it down in the
kernel, but I'd like some direction.

--

Robert Engels
OptionsCity Software Inc.
Chicago, IL
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