That's an odd failure, since the time taken should be much closer to 0 on failure and much closer to 0.5 seconds on success. Something must have somehow managed to pause the process or otherwise interrupt CPU time accounting for almost exactly 0.25 seconds during the execution of this bit of code:

$t = microtime( 1 ) + 0.5;
while ( microtime( 1 ) < $t ) {
    # Waste CPU cycles
}

Is this reliably flaky, or did it happen just the once? See also T111075#1593802.

It only happened once. Thx for pointing out the code, so one of those is real time, the other is CPU time. As in this case only real >= CPU is always true, I think that qualifies as flaky. Though I have no good idea how to ensure we spend x amount of cpu time on any system (no matter how fast).

https://integration.wikimedia.org/ci/job/mwext-testextension-hhvm-composer/1070/console

Failed asserting that 0.168662834 is greater than 0.25.

Change 270023 had a related patch set uploaded (by Anomie):
Waste CPU cycles, not wall clock time

https://gerrit.wikimedia.org/r/270023

Change 270023 merged by jenkins-bot:
Waste CPU cycles, not wall clock time

https://gerrit.wikimedia.org/r/270023