Re: [PATCH] Count number of logical processors sharing L2 cache

[Paul Eggert <eggert at cs dot ucla dot edu>]

On 05/31/2016 08:26 AM, Carlos O'Donell wrote:
> Most reasonable administrators would use CAT to limit one particular
> workload and leave the CAT setting until the workload changed.

This sounds like a reasonable assumption. That being said, on fancier 
processors (Intel Xeon E5 v4), the CAT COS tables can change at 
run-time. Plus, even on less-fancy processors I would not be surprised 
if some kernels blindly overwrite a CPU's COSid on context switch, so 
that the last logical thread to switch into a CPU socket determines the 
CAT COS tables for all physical threads currently on that socket. Either 
way, it would be wise to document the assumption, as it reminds me of 
other assumptions (e.g., "the time zone database does not change during 
execution of the program") that formerly were quite reasonable but now 
show signs of strain.