1.3. Virtualization Performance Features and Improvements

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Virtualization Performance Improvements in Red Hat Enterprise Linux 7

The following features improve virtualization performance in Red Hat Enterprise Linux 7:
Automatic NUMA Balancing
Automatic NUMA balancing improves the performance of applications running on NUMA hardware systems, without any manual tuning required for Red Hat Enterprise Linux 7 guests. Automatic NUMA balancing moves tasks, which can be threads or processes, closer to the memory they are accessing. This enables good performance with zero configuration. However, in some circumstances, providing more accurate guest configuration or setting up guest to host affinities for CPUs and memory may provide better results.
For more information on automatic NUMA balancing, see Section 9.2, “Automatic NUMA Balancing”.
VirtIO models
Any virtual hardware that has the virtio model does not have the overhead of emulating the hardware with all its particularities. VirtIO devices have low overhead thanks to the fact that they are designed specifically for use in Virtualization environments. However not all guest operating systems support such models.
Multi-queue virtio-net
A networking approach that enables packet sending/receiving processing to scale with the number of available vCPUs of the guest.
For more information on multi-queue virtio-net, see Section 5.4.2, “Multi-Queue virtio-net”.
Bridge Zero Copy Transmit
Zero copy transmit mode reduces the host CPU overhead in transmitting large packets between a guest network and an external network by up to 15%, without affecting throughput. Bridge zero copy transmit is fully supported on Red Hat Enterprise Linux 7 virtual machines, but disabled by default.
For more information on zero copy transmit, see Section 5.4.1, “Bridge Zero Copy Transmit”.
APIC Virtualization (APICv)
Newer Intel processors offer hardware virtualization of the Advanced Programmable Interrupt Controller (APICv). APICv improves virtualized AMD64 and Intel 64 guest performance by allowing the guest to directly access the APIC, dramatically cutting down interrupt latencies and the number of virtual machine exits caused by the APIC. This feature is used by default in newer Intel processors and improves I/O performance.
EOI Acceleration
End-of-interrupt acceleration for high bandwidth I/O on older chipsets without virtual APIC capabilities.
Multi-queue virtio-scsi
Improved storage performance and scalability provided by multi-queue support in the virtio-scsi driver. This enables each virtual CPU to have a separate queue and interrupt to use without affecting other vCPUs.
For more information on multi-queue virtio-scsi, see Section 7.4.2, “Multi-Queue virtio-scsi”.
Paravirtualized Ticketlocks
Paravirtualized ticketlocks (pvticketlocks) improve the performance of Red Hat Enterprise Linux 7 guest virtual machines running on Red Hat Enterprise Linux 7 hosts with oversubscribed CPUs.
Paravirtualized Page Faults
Paravirtualized page faults are injected into a guest when it attempts to access a page swapped out by the host. This improves KVM guest performance when host memory is overcommitted and guest memory is swapped out.
Paravirtualized Time vsyscall Optimization
The gettimeofday and clock_gettime system calls execute in the user space through the vsyscall mechanism. Previously, issuing these system calls required the system to switch into kernel mode, and then back into the user space. This greatly improves performance for some applications.

Virtualization Performance Features in Red Hat Enterprise Linux

  • CPU/Kernel
    • NUMA - Non-Uniform Memory Access. See Chapter 9, NUMA for details on NUMA.
    • CFS - Completely Fair Scheduler. A modern class-focused scheduler.
    • RCU - Read Copy Update. Better handling of shared thread data.
    • Up to 160 virtual CPUs (vCPUs).
  • Memory
    • huge pages and other optimizations for memory-intensive environments. See Chapter 8, Memory for details.
  • Networking
    • vhost-net - A fast, kernel-based VirtIO solution.
    • SR-IOV - For near-native networking performance levels.
  • Block I/O
    • AIO - Support for a thread to overlap other I/O operations.
    • MSI - PCI bus device interrupt generation.
    • Disk I/O throttling - Controls on guest disk I/O requests to prevent over-utilizing host resources. See Section 7.4.1, “Disk I/O Throttling” for details.


For more details on virtualization support, limits, and features, see the Red Hat Enterprise Linux 7 Virtualization Getting Started Guide and the following URLs:
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