Re: [Xen-devel] rdtscP and xen (and maybe the app-tsc answer I've been looking for)

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 21/09/2009 16:41, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

>> Keir, is there any reason that consistent_tscs shouldn't
>> default to enabled?
> 
> There was some question whether it means what you think it means across NUMA
> nodes. If we are sure that it does guarantee consistency across NUMA nodes
> -- or if we don't care about that -- then it can be enabled by default.

There is a question mark over this, since it's not really clear what the
CONSTANT_TSC feature flag actually means. For example, it is set if
CPUID:0x80000007:EDX:8 is set, and that flag merely means that this
particular core's TSC rate is invariant across all Cx/Px/Tx power-saving
states. It doesn't directly say anything about TSC consistency across cores
or sockets unless we are prepared to assume a couple of things: primarily
that all packages run their TSCs at the same rate, and that they are clocked
from the same mainboard oscillator. Is that reasonable to assume? We at
least know the latter is not likely to be true for big-iron NUMA systems,
across NUMA nodes.

 -- Keir



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