Marcelo Tosatti wrote:
Avi, I don't think this causes such a huge performance regression. NOHZ makes the frequency of timer reads go down significantly.
Have we yet determined why the TSC is so unstable in the first place? In theory, it should be relatively stable on single-node Intel and Barcelona chips.
Regards, Anthony Liguori
As for constant tick guests, well, the impact will be similar to changing to SMP, since those fallback to ACPI timer anyway now. The C2 emulation is required by Ubuntu 7.10 for example, which refuses to process the CST notification. Stock Linux kernels (as old as 2.6.20) do mark C2 invalid upon the CST notification.
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