Re: [RFC 2/2] Introduce sysctl(s) for the migration costs

[Steven Sistare]

On 2/9/2018 12:08 PM, Mike Galbraith wrote:
> 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.

I was imprecise.  The cache-loss component of cost as represented by 
sched_migration_cost should be 0 in this case.  The cost of the machinery
is non-zero and remains in the code, and can still prevent migration.

>>  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.

Yes, yes and yes. I cannot make the original tunable less bogus.  Using a smaller
cost for closer caches still makes logical sense and is supported by the data.

- Steve