Community: Testing Linux in the Real World

By John Telford

I’m delighted seeing efforts such as the Linux Test Project and
VA Cerberus Test Control System focusing on testing Linux.

Several years ago I began working with an early Unix symmetric
multiprocessing (SMP) vendor. My marching orders included creating
a team responsible for testing the porting of BSD 4.2 Unix.


The first order of business was gaining the trust of the
world-class Unix gurus doing the porting. Some of them were still
reeling from bad experiences with traditional Quality Assurance
(QA) groups. Instead of having the porting team throw chunks of
code over the wall and the testing team throwing it back saying its
dead beef, members of the porting and testing teams work

The testing team is the loyal opposition. Their obsession is
creating successful test cases showing the Unix port isn’t work as
advertised. Once that becomes very difficult, we all win.

When a successful test case surfaces a defect in the Unix port,
a member of the porting and testing teams get together and resolve
the problem. The first issue resolved is insuring the test case is
working as advertised. Once they agree it’s sane, they work
together finding the defect in the Unix port.

Successful Test Case

A successful test case shows something isn’t working. It’s far
more valuable than one showing something is working. The time when
creating successful test cases becomes difficult, is when whatever
we’re creating just may be ready for the real world.

One way of going about crafting test cases is start by showing
there’s life at the “center” of the functional range of whatever
we’re testing. It soon becomes difficult creating successful test
cases at this level of testing.

The next test cases aims at just inside the functional
boundaries. It’s less difficult creating test successful at this
level of testing than in the “center.”

The easiest testing level for creating a successful test case is
aiming outside the functional boundaries.

These simple concepts aren’t simple to manifest. They’re
counter-intuitive. Surfacing defects in one’s creation just isn’t
in the mindset of most developers, test creators, or their

Testing SMP

Testing SMP requires something more than running a single
instance of test cases. The idea is to surface subtle timing
problems, lock defects, etc. Seldom does a single instance do

One way of doing it is running multiple instances of several
test cases. Vary the number of instances from low to beyond
saturation, and randomly vary the timing between each instance.
This simple process generates something like an ocean tide.

The testing tide begins gently when it comes in and works up to
giving the platform under test a pounding at high tide. It lets up
when the tide goes back out. Varying the number and timing of test
cases insures the testing tide is seldom the same. This simple
mechanism is remarkable. It sometimes surfaces deeply rooted
defects. There is a down side. It’s virtually impossible to repeat
the exact testing sequences leading up to a defect surfacing.

This testing philosophy works for non-SMP too.

Platform Testing

The Unix porting and testing teams enjoyed success using this
testing philosophy. Creating successful test cases was becoming

Meanwhile manufacturing was busy assembling parts and sparking
life into platforms. They were looking for ways of burning-in,
stressing, and accelerating infant mortality among their creations.
They saw how well testing was going with the Unix porting and
testing teams and wondered if it could be adapted to manufacturing

The testing team stepped up to the challenge by expanding its
scope to creating test suites for manufacturing.

The core testing philosophy remains the same. The added twist is
creating tides and cross currents that flow through every platform
data path and orifice.

Once again the simple mechanisms of varying the number and
timing of test cases successfully surfaces deeply rooted defects.
Now the defects seeing the light of day are in processors and
controller chips, as well as in the kernel, device drivers, and

Sometimes deeply rooted defects didn’t surface until a day or so
after testing started. The testing tides had to be just right, sort
of like the phase of the moon.

Manufacturing liked the test suites, except for the downside of
being unable to exactly repeat the testing sequence leading up to a
defect surfacing.

Bottom Line

The testing efforts I’m describing helped insure we could be
proud of the quality of product shipping. About the best complement
I heard from a customer was she knew how to reboot all the other
Unix boxes except ours because it just didn’t go down.

Maybe the Linux testing efforts and Linux can benefit from our


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