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Prime95 workload causing Intel Skylake CPUs to freeze

It is recently discovered that the sixth-generation Intel Skylake processors have a bug which can be triggered when running Prime95.

The issue was initially discovered in December 2015 where testers were running and documenting the problem in the Great Internet Mersenne Prime Search (GIMPS) forum. The first report indicates that the bug gets triggered when running the 768K test on the i7 6700K at stock clock speeds with all threads. The motherboard vendor ASROCK initially gave a solution to increase the VCORE and try using Prime95’s 28.7 version which uses AVX2 instructions. Turns out, the bug was not that simple.

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Many users with the Skylake CPU users have collaborated to find out the actual problem- different motherboards, power supplies, operating systems, voltage levels, clock speed and even memory kits. Based on some reports, the problem could be fixed by disabling Hyper-Threading via the BIOS.

Intel issued a statement which indicated that the problem can be solved via a BIOS update:

Hello All,
Intel has identified an issue that potentially affects the 6th Gen Intel® Core™ family of products. This issue only occurs under certain complex workload conditions, like those that may be encountered when running applications like Prime95. In those cases, the processor may hang or cause unpredictable system behavior. Intel has identified and released a fix and is working with external business partners to get the fix deployed through BIOS.

The bug causes the system to crash with Windows and Linux operating systems.

This isn’t the first time bugs were found and it’s not limited just with Intel. AMD’s first Phenom lineups suffered TLB bug while Intel’s first TSX implementation was disabled (Transactional Synchronization Extension) via an update. Even in the early 90’s Pentium lineup was under heavy criticism due to an FDIV bug where it replied with incorrect floating-point in certain specific workload by 0.000061. AMD Piledriver architecture was known to have issues with the 256-AVX instruction sets which cause cycle delay when running multiple instructions.

Prime95 786K trigger

Intel does documents and lists such errors as it did with Intel Haswell Lineups. However, as one would expect some bugs are found after a certain period depending on the complex calculation. Triggering the bug requires some work and it seems understandable by the usual benchmarks and workloads done by both users and reviewers wouldn’t have found this in most cases. After downloading Prime95, you’ll need to go to Advanced->Test-> use the value “1492209” under ‘Exponent to test’. That said, systems with Skylake CPUs would take a certain time to freeze- minutes or even hours.

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