KVM now includes various modifications which reduce the workload for interrupt handling and therefore enhance performance (1, 2, 3). Another addition to the kernel is IOMMU Groups, which improve the isolation of PCI and PCIe devices using I/O virtualisation technologies such as AMD-Vi and Intel's VT-d. The IOMMU Groups feature is also the basis for the VFIO (Virtual Function I/O) userspace driver framework (1, 2, 3); mainly intended for KVM, this feature is designed to pass through PCI and PCIe devices to guests, allowing them to access these devices at low latency and high data throughput levels, and without any risk to the host. Details on VFIO are available in the documentation and in an article on LWN.net. Extensions to provide VFIO via QEMU are still in development.
The TCP Small Queues (TSQ) feature makes Linux 3.6 use small buffers of no more than 128KB per network socket by default. Like various other recent kernel modifications, this measure is designed to avoid bufferbloat – a term used for problems such as high network latencies and disrupted connections caused by too much buffering during data transfers. According to test results provided by the responsible developer, the smaller buffers per socket don't affect data throughput; if required, administrators can use the /proc/sys/net/ipv4/tcp_limit_output_bytes proc file to adjust the value at runtime.
The kernel now includes client-side support for TCP Fast Open (TFO) (1, 2, 3, 4, 5); server-side support is planned to follow in Linux 3.7. TFO is an experimental TCP extension that has not yet been approved by the Internet Assigned Numbers Authority (IANA). It is designed to reduce the overhead when establishing a connection, which is supposed to speed up HTTP connections.
The kernel developers have merged a range of additions and bug fixes into Linux 3.6 aimed at improving support for Apple MacBook laptops released in July (such as the MacBook Pro Retina). Not all of the additions required for the new MacBooks have yet found their way into Linux 3.6. In addition, on some models, this version still shows problems during initialisation or use of the graphics hardware.
The Radeon graphics driver now activates the fastest PCIe 2.0 transfer modes where these are supported by the hardware. The audio drivers now support Intel's Haswell processors and associated Lynx Point platform controller hubs (PCHs), which Intel appears to be planning on releasing over the next year (1, 2). A driver for Lenovo's ThinkPad USB keyboard with trackpoint has been merged into the input subsystem.
The bnx2x driver, which handles various Ethernet chips from Broadcom, now supports the Energy-Efficient Ethernet (EEE) standard that is specified in IEEE 802.3az. Users can read or set EEE parameters via the recently released ethtool 3.5.