RE: [PATCH 1/2] x86/resctrl, Documentation: Keep mbm_assign_mode "default" on boot

[Luck, Tony]

> ABMC mode (also referred to as mbm_event mode) relies on ABMC counters,
> and this system provides only 32 such counters.
>
> By comparison, the default mode uses RMIDs, with up to 4096 RMIDs
> available. However, even though the RMID space is much larger, the
> hardware can track only 64 RMIDs concurrently. As a result, if pqos
> attempts to monitor more than 64 groups, the reported bandwidth
> measurements may become inaccurate.
>
> That said, I have not been able to reproduce any bandwidth inaccuracies
> on the systems I tested. My point is that this may not be a significant
> issue for the vast majority of users

How did you try to reproduce? It seems that it should be trivial to create
more than 64 monitor groups with active tasks assigned to each group.
That ought to show the issue quickly.

What does happen when I'm reading the memory bandwidth from a
monitor group and the underlying counter for that RMID is stolen for
use by another group, and then later this group steals a counter from
someone else?

I'm expecting "Unavailable" while there is no counter. But I'm not sure
what happens when a counter is reacquired.

1) Does it start over from zero?
	Seems a sane option.
2) Or does the monitor group see whatever the counter was at
when being used by some other group?
	Seems less sane.

-Tony