Tests show how much Meltdown fixes will hit Linux system performance

Now that the initial shock about the Spectre and Meltdown chip vulnerabilities has died down, the focus is very much on getting the problems sorted. As has been noted already, there has been concern about the impact on performance that the bug fixes will bring.

Intel has been eager to downplay any suggestion of major slowdown, but the exact performance hit will vary from system to system depending on the tasks being performed. Brendan Gregg -- a Netflix engineer whose work involves large scale cloud computing performance -- has conducted some tests into the impact patches will have on Linux systems, concluding that "patches that workaround Meltdown introduce the largest kernel performance regressions I've ever seen."

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Gregg has created a "simple microbenchmark" (for which he has provided the source code) which he used to test five factors he believes are key. He is looking specifically at the Linux kernel page table isolation (KPTI) patches, but points out that there are three other layers of overhead to consider as well (Intel microcode updates, cloud provider hypervisor changes, and retpoline compiler changes).

He summarizes five key factors that are involved in KPTI overheads:

Brendan's findings suggest that Linux systems can expect to see a slowdown of between 0.1 and six percent. For small scale tasks, this will be fairly insignificant, but the same cannot be said for bigger tasks or large scale Linux systems. It's not all bad news, though. Intel has stressed that it should be possible to mitigate against any performance hit, and Gregg tends to agree. "I'm expecting we'll take that down to less than 2 percent with tuning," Brendan adds.

For a detailed breakdown of Gregg's findings, take a look at his blog post.