Bug#941284: Wishlist/RFC: Use CONFIG_HZ=100 in linux-image-cloud-*

From	Nicholas D Steeves <nsteeves@gmail.com>
Newsgroups	linux.debian.bugs.dist, linux.debian.kernel
Subject	Bug#941284: Wishlist/RFC: Use CONFIG_HZ=100 in linux-image-cloud-*
Date	2019-09-28 06:20 +0200
Message-ID	<yLcn7-5pE-1@gated-at.bofh.it> (permalink)
References	<yL5lD-SU-3@gated-at.bofh.it> <yL5lD-SU-3@gated-at.bofh.it> <yL745-207-3@gated-at.bofh.it> <yL5lD-SU-3@gated-at.bofh.it> <yL745-207-3@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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Noah Meyerhans <noahm@debian.org> writes:

> On Fri, Sep 27, 2019 at 01:15:29PM -0700, Flavio Veloso wrote:
>> Since linux-image-cloud-* packages are created for cloud environments --
>> read: servers which do not need desktop-level responsiveness --, wouldn't it
>> be beneficial to build the kernels with CONFIG_HZ set to 100?
>

I wonder if the upstream docstring could be improved, because to my mind
100hz only makes sense for throughput-limited workloads, or for big SMP
systems. eg: when you have one 100hz timer running on each of 32 cores
the effect on latency is more like a ~3200hz timer than a 100hz one.

Also, network latency is a big deal now, so I'm not sure if this
docstring is accurate anymore.

> For what it's worth, the Amazon Linux 2 4.14.x kernel also ships with
> CONFIG_HZ=250. We obviously don't need to use the same settings, but
> that kernel specifically targets cloud deployments, and its maintainers
> do not see a need to set CONFIG_HZ=100.
>
> I don't know what considerations were taken into account when choosing
> the value for that variable at AWS.
>

Maybe something like socket activation, subprocess or thread creation
latency?  Or maybe because it's useful to have a 250hz timer when your
vhost only has one vcpu?

'just some non-authoritative thoughts.  I'm sure Ben can say for sure
:-)

Take care!
Nicholas

Thread

Bug#941284: Wishlist/RFC: Use CONFIG_HZ=100 in linux-image-cloud-* Flavio Veloso <flaviovs@magnux.com> - 2019-09-27 22:50 +0200
  Bug#941284: Wishlist/RFC: Use CONFIG_HZ=100 in linux-image-cloud-* Noah Meyerhans <noahm@debian.org> - 2019-09-28 00:40 +0200
    Bug#941284: Wishlist/RFC: Use CONFIG_HZ=100 in linux-image-cloud-* Nicholas D Steeves <nsteeves@gmail.com> - 2019-09-28 06:20 +0200
      Bug#941284: Wishlist/RFC: Use CONFIG_HZ=100 in linux-image-cloud-* Flavio Veloso <flaviovs@magnux.com> - 2019-10-01 01:40 +0200

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