Re: [patch] Performance Counters for Linux, v4

[Corey Ashford <cjashfor@linux.vnet.ibm.com>]

Andi Kleen wrote:
> Paul Mackerras <paulus@samba.org> writes:
>> The perf counter subsystem will, in Ingo's design, naturally try to
>> schedule as many counters and groups on as it can.  Given a list of
>> counters/groups, it could start with the first and keep on trying to
>> add counters or groups while it can, essentially trying all possible
>> combinations until it either fills up all the hardware counters or
>> exhausts the possible combinations.  If it moves all the
>> counters/groups that do fit on up to the head of the list, and then
>> rotates them to the back of the list when the timeslice expires, that
>> would probably be OK.  In fact the computation about what set of
>> counters/groups to put on should be done when adding/removing a
>> counter/group and when the timeslice expires, rather than at context
>> switch time.  (I'm talking about the list of part-time counters/groups
>> here, of course.)
> 
> One issue is that PMU counts can cover more than one CPU. One example
> for this are the Uncore events on Nehalem (which cover a whole socket)
> or when you are in AnyThreads monitoring mode (then you get events
> from both SMT siblings in a core)
> 
> With that you would need to examine other CPU's state at context switch
> time. Probably not a good idea for scalability.
> 
> -Andi
> 

Over time, it seems clear that we will see multi-core processor designs 
with increasingly large uncore/nest facilities, so this could become 
more and more of an issue.

- Corey

Corey Ashford
Software Engineer
IBM Linux Technology Center, Linux Toolchain
Beaverton, OR
503-578-3507
cjashfor@us.ibm.com