Re: [PATCH v3 00/21] Cache Aware Scheduling

[Peter Zijlstra]

On Fri, Feb 20, 2026 at 03:29:41AM +0000, Qais Yousef wrote:

> What's the reason wake up doesn't have the latest info? Is this a limitation of
> these large systems where stats updates are too expensive to do? Is it not
> fixable at all?

Scalability is indeed the main problem. The periodic load-balancer, by
virtue of being 'slow' has two advantages:

 - the cost of aggregating the numbers is amortized by the relative low
   frequency of aggregation

 - it can work with averages; it is less concerned with immediate
   spikes.

This obviously has the exact inverse set of problems in that it is not
able to deal with immediate/short term issues.


Anyway, we're already at the point where EAS wakeup path is getting far
too expensive for the current set of hardware. While we started with a
handful of asymmetric CPUs, we're now pushing 32 CPUs or so.

(Look at Intel Nova Lake speculation online, that's supposedly going to
get us 2 dies of 8P+16E with another 4 bonus weaklings on the south
bridge or something, for a grand total of 52 asymmetric CPUs of 3 kinds)


Then consider:

 - Intel Granite Rapids-SP at 8*86 cores for 688 cores / 1376 threads. 

 - AMD Prometheus at 2*192 cores with 384 cores / 768 threads.  These
   are silly number of CPUs. 

 - Power10, it is something like 16 sockets, 16 cores per socket, 8
   threads per core for a mere 2048 threads.

Now, these are the extreme end of the spectrum systems, 'nobody' will
actually have them, but in a few generations they'll seem small again.


So whatever we build now, will have to deal with silly numbers of CPUs.