From: Juergen Gross <jgross@suse.com>
To: Krister Johansen <kjlx@templeofstupid.com>,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
Jan Beulich <jbeulich@suse.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
Marcelo Tosatti <mtosatti@redhat.com>,
Anthony Liguori <aliguori@amazon.com>,
David Reaver <me@davidreaver.com>,
Brendan Gregg <brendan@intel.com>
Subject: Re: [PATCH linux-next v3] x86/xen/time: prefer tsc as clocksource when it is invariant
Date: Tue, 17 Jan 2023 13:12:34 +0100 [thread overview]
Message-ID: <eda8d9f2-3013-1b68-0df8-64d7f13ee35e@suse.com> (raw)
In-Reply-To: <20221216162118.GB2633@templeofstupid.com>
[-- Attachment #1.1.1: Type: text/plain, Size: 1962 bytes --]
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
[-- Attachment #1.1.2: OpenPGP public key --]
[-- Type: application/pgp-keys, Size: 3149 bytes --]
[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 495 bytes --]
next prev parent reply other threads:[~2023-01-17 12:12 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-16 16:21 [PATCH linux-next v3] x86/xen/time: prefer tsc as clocksource when it is invariant Krister Johansen
2023-01-17 12:12 ` Juergen Gross [this message]
2023-02-13 8:18 ` Juergen Gross
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=eda8d9f2-3013-1b68-0df8-64d7f13ee35e@suse.com \
--to=jgross@suse.com \
--cc=aliguori@amazon.com \
--cc=boris.ostrovsky@oracle.com \
--cc=bp@alien8.de \
--cc=brendan@intel.com \
--cc=dave.hansen@linux.intel.com \
--cc=hpa@zytor.com \
--cc=jbeulich@suse.com \
--cc=kjlx@templeofstupid.com \
--cc=linux-kernel@vger.kernel.org \
--cc=me@davidreaver.com \
--cc=mingo@redhat.com \
--cc=mtosatti@redhat.com \
--cc=tglx@linutronix.de \
--cc=x86@kernel.org \
--cc=xen-devel@lists.xenproject.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).