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Performance Evaluation of AMD RVI Hardware Assist










We
recently released a whitepaper that demonstrates huge performance gains provided
by VMware ESX server on latest third generation AMD Opteron™ processors. These
processors introduce AMD’s
second generation hardware support for
virtualization that incorporates memory management unit (MMU) virtualization,
called Rapid Virtualization Indexing™ (RVI). Intel has also announced MMU
virtualization support in their “Nehalem” processors called Extended Page
Tables™ (EPT).
ESX
has been adopting these technologies as they are introduced, and many workloads
see performance benefits as a result. The benefits seen can include higher
throughput and lower CPU utilization, improving user experience and freeing up
servers for greater consolidation. The paper can be found here: Performance Evaluation of AMD RVI Hardware Assist

The
performance gains observed in this paper were up to 42% for MMU intensive
benchmarks and up to 500% for MMU-intensive microbenchmarks compared to
software-only virtualization. We also observed that although RVI increases
memory access latencies for a few workloads, this cost can be reduced by
effectively using large pages in the guest and the hypervisor.  For optimal performance, ESX aggressively
tries to use large pages for its own memory when RVI is used. 

Prior
to the introduction in 2006 of first-generation hardware support for x86
virtualization, AMD-V from AMD and Intel VT-x from Intel, the VMware virtual
machine monitor (VMM) relied upon software-only techniques for virtualizing x86
processors.

We
used:

  • Binary Translation (BT) for
    instruction-set virtualization

  • Shadow Paging for MMU virtualization

  • Device Emulation for device virtualization.

With
the advent of first-generation hardware support, the VMM could make use of the
hardware features for instruction-set virtualization. However, MMU and device
virtualization were still done in software. Now with the introduction of
second-generation virtualization hardware support, the VMM can take advantage
of hardware-assist for both instruction-set and MMU virtualization. MMU
virtualization allows the guest to access only those memory locations that
belong to it. In software MMU virtualization, this requires the VMM to
intercept guest execution when the guest updates its virtual memory data
structures (page tables). In hardware MMU virtualization the hardware provides
a mechanism by which the VMM no longer needs to intercept guest execution
during page table updates. This results in significant performance improvements
for workloads that stress the x86 MMU.
Continue reading here.










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