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SubjectRe: [RFC 2/2] Introduce sysctl(s) for the migration costs
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On Fri, 2018-02-09 at 11:10 -0500, Steven Sistare wrote:
> On 2/8/2018 10:54 PM, Mike Galbraith wrote:
> > On Thu, 2018-02-08 at 14:19 -0800, Rohit Jain wrote:
> >> This patch introduces the sysctl for sched_domain based migration costs.
> >> These in turn can be used for performance tuning of workloads.
> >
> > With this patch, we trade 1 completely bogus constant (cost is really
> > highly variable) for 3, twiddling of which has zero effect unless you
> > trigger a domain rebuild afterward, which is neither mentioned in the
> > changelog, nor documented.
> >
> > bogo-numbers++ is kinda hard to love.
>
> Yup, the domain rebuild is missing.
>
> I am no fan of tunables, the fewer the better, but one of the several flaws
> of the single figure for migration cost is that it ignores the very large
> difference in cost when migrating between near vs far levels of the cache hierarchy.
> Migration between CPUs of the same core should be free, as they share L1 cache.
> Rohit defined a tunable for it, but IMO it could be hard coded to 0.

That cost is never really 0 in the context of load balancing, as the
load balancing machinery is non-free.  When the idle_balance() throttle
was added, that was done to mitigate the (at that time) quite high cost
to high frequency cross core scheduling ala localhost communication.

> Migration
> between CPUs in different sockets is the most expensive and is represented by
> the existing sysctl_sched_migration_cost tunable. Migration between CPUs in
> the same core cluster, or in the same socket, is somewhere in between, as
> they share L2 or L3 cache. We could avoid a separate tunable by setting it to
> sysctl_sched_migration_cost / 10.

Shrug. It's bogus no mater what we do. Once Upon A Time, a cost
number was generated via measurement, but the end result was just as
bogus as a number pulled out of the ether.  How much bandwidth you have
when blasting data to/from wherever says nothing about misses you avoid
vs those you generate.

-Mike

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