Your CPU May Have Slowed Down on Wednesday
The plot below shows the throughput of filling a region of the given size (varying on the x-axis) with zeros1 on Skylake (and Ice Lake in the second tab).
The two series were generated under apparently identical conditions: the same binary on the same machine. Only the date the benchmark was run varies. That is, on Tuesday (June 7th) filling with zeros is substantially faster than the same benchmark on Wednesday, at least when the region no longer fits in the L2 cache2.
What’s going on here? Are my Skylake and Ice Lake hosts simply work-weary by Wednesday and don’t put in as much effort? Is there a new crypto-coin based on who can store the most zeros and this is a countermeasure to avoid ballooning CPU prices in the face of this new workload?
These hosts run Ubuntu 20.04 and on Wednesday June 8th an update to the intel-microcode OS package was released. After a reboot3, this loads the CPU with new microcode that causes the behavior shown above. Specifically, this microcode4 disables the hardware zero store optimization we discussed in a previous post. It was disabled to mitigate CVE-2020-24512 further described5 in Intel security advisory INTEL-SA-00464.
