Messages in this thread | | | Date | Tue, 17 Jan 2023 13:12:34 +0100 | Subject | Re: [PATCH linux-next v3] x86/xen/time: prefer tsc as clocksource when it is invariant | From | Juergen Gross <> |
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On 16.12.22 17:21, Krister Johansen wrote: > Kvm elects to use tsc instead of kvm-clock when it can detect that the > TSC is invariant. > > (As of commit 7539b174aef4 ("x86: kvmguest: use TSC clocksource if > invariant TSC is exposed")). > > Notable cloud vendors[1] and performance engineers[2] recommend that Xen > users preferentially select tsc over xen-clocksource due the performance > penalty incurred by the latter. These articles are persuasive and > tailored to specific use cases. In order to understand the tradeoffs > around this choice more fully, this author had to reference the > documented[3] complexities around the Xen configuration, as well as the > kernel's clocksource selection algorithm. Many users may not attempt > this to correctly configure the right clock source in their guest. > > The approach taken in the kvm-clock module spares users this confusion, > where possible. > > Both the Intel SDM[4] and the Xen tsc documentation explain that marking > a tsc as invariant means that it should be considered stable by the OS > and is elibile to be used as a wall clock source. > > In order to obtain better out-of-the-box performance, and reduce the > need for user tuning, follow kvm's approach and decrease the xen clock > rating so that tsc is preferable, if it is invariant, stable, and the > tsc will never be emulated. > > [1] https://aws.amazon.com/premiumsupport/knowledge-center/manage-ec2-linux-clock-source/ > [2] https://www.brendangregg.com/blog/2021-09-26/the-speed-of-time.html > [3] https://xenbits.xen.org/docs/unstable/man/xen-tscmode.7.html > [4] Intel 64 and IA-32 Architectures Sofware Developer's Manual Volume > 3b: System Programming Guide, Part 2, Section 17.17.1, Invariant TSC > > Signed-off-by: Krister Johansen <kjlx@templeofstupid.com> > Code-reviewed-by: David Reaver <me@davidreaver.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Juergen
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