Testing Intel Raptor Lake With PCID Disabled & A Move Made By FreeBSD Last Year

Written by Michael Larabel in Intel on 22 May 2023 at 06:11 AM EDT. Add A Comment
With Linux going to disable PCID support on Intel Alder Lake and Raptor Lake while waiting for mitigated microcode due to a CPU bug, I was curious if this disabling of Process Context Identiifiers would have any overall performance implications. So I ran some benchmarks this weekend.

From yesterday's article on this Linux patch for disabling PCID on Alder Lake and Raptor Lake, an interesting anecdote was raised by a Phoronix reader. While only this week were Intel engineers preparing the Linux kernel patch for this change, on the FreeBSD side they made a similar kernel change all the way back at the end of 2022. Phoronix reader jrtc27 pointed out this patch:
amd64: for small cores, use (big hammer) INVPCID_CTXGLOB instead of INVLPG

A hypothetical CPU bug makes invalidation of global PTEs using INVLPG in pcid mode unreliable, it seems. The workaround is applied for all CPUs with small cores, since we do not know the scope of the issue, and the right fix.

Anyhow, with the Linux patch for disabling PCID on Alder Lake and Raptor Lake, I was curious if on an otherwise out-of-the-box Linux setup if it would result in any performance loss for end-user workloads. Long story short, it's not.

Intel 13900K cpuinfo output

I ran some benchmarks with the Intel Core i9 13900K on Ubuntu 23.04 and then again with the "nopcid" kernel option to have the same effect as the pending Linux kernel change.
Geometric Mean Of All Test Results benchmark with settings of Result Composite, NO PCID Raptor Lake Test. PCID Active was the fastest.

Across 83 benchmarks overall there was no noticeable change in performance when running the i9-13900K Raptor Lake with PCID forced off.
NO PCID Raptor Lake Test

In only a handful of those 83 tests was there some minor movement when turning off PCID support. All the benchmarks in full and system details for those curious via this result page.

So long story short, it doesn't look like PCID being turned off for Alder Lake and Raptor Lake until there is updated CPU microcode in the future for this bug will lead to any noticeable performance changes for end-users based on this initial testing.
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Michael Larabel

Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 20,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter, LinkedIn, or contacted via MichaelLarabel.com.

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