1.3. Virtualization Performance Features and Improvements

Virtualization Performance Improvements 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.