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[Bug 1564949] Re: Severe latency/skew on AMD Opetron processor
$ cat /etc/lsb-release
DISTRIB_DESCRIPTION="Ubuntu Xenial Xerus (development branch)"
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Severe latency/skew on AMD Opetron processor
Status in linux package in Ubuntu:
Discovered this while doing pre-release certification testing for
16.04 on an HP ProLiant DL385p Gen8 with an AMD Opteron 6320 8-core
I have some code that essentially does this: And note, I am NOT a C
programmer, I know enough C to read it and do some minor things, and I
once knew C++ fairly well, about 10 years ago...
where tval_start and tval_stop are timeval structs and sleeptime is
Once it gets the start and stop it finds the delta minus the sleep
In a perfect world, for example, start time would be 123456.123 and
end time would be 123516.123 and the delta between them minus the 60
seconds of sleep would be 0.
Of course, that isn't how it works in reality so the delta may be a
few microseconds here and there depending on what else the kernel is
doing at any given moment. The following, however, are on essentially
idle Xenial systems (only processes running are whatever Ubuntu Server
runs by default, nothing really taxing going on).
On my Skylake i7 with Xenial, the time differences are never more than
a few 10,000ths of a second: (kernel 4.4.0-15.31)
Testing clock direction for 5 minutes...
PASSED: Iteration 0 delta: 0.000109
PASSED: Iteration 1 delta: 0.000068
PASSED: Iteration 2 delta: 0.000107
PASSED: Iteration 3 delta: 0.000216
PASSED: Iteration 4 delta: 0.000089
On a zVM instance (kernel 4.4.0-16.32) it's even better:
PASSED: Iteration 0 delta: 0.000058
PASSED: Iteration 1 delta: 0.000058
PASSED: Iteration 2 delta: 0.000074
PASSED: Iteration 3 delta: 0.000052
PASSED: Iteration 4 delta: 0.000062
But on an AMD cpu with Xenial (the only AMD CPU I have access to), the
difference is always in the 10ths of a second, sometimes even several
seconds... in other words, I've seen up to a 7.9 second delta with
this code. Here's one run that shows 3 seconds in one iteration:
FAILED: Iteration 0 delta: 3.057980
FAILED: Iteration 1 delta: 0.225712
FAILED: Iteration 2 delta: 0.241468
FAILED: Iteration 3 delta: 0.229084
FAILED: Iteration 4 delta: 0.223933
I ran a second run on the AMD cpu and the latency was all over the place:
FAILED: Iteration 0 delta: 9.302149
FAILED: Iteration 1 delta: 0.624466
FAILED: Iteration 2 delta: 1.644834
FAILED: Iteration 3 delta: 1.011474
FAILED: Iteration 4 delta: 0.923033
After a discussion with cking and apw, deviations of as seen on the
Intel and s390 CPUs are about what we should expect to see depending
on what the system is doing at the moment gettimeofday() is executed.
However, on the AMD CPU, differences of up to 9 seconds or more are
NOT expected and highly irregular.
Colin said he tested this on an AMD C60 CPU and got numbers inline
with the Skylake and s390 chips and could not reproduce the times I am
seeing on the Opteron.
$ cat /proc/version_signature
Ubuntu 4.4.0-15.31-generic 4.4.6
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