Chapter 2. Kernel

The kernel shipped in Red Hat Enterprise Linux 6.4 includes several hundred bug fixes for, and enhancements to, the Linux kernel. For details concerning important bugs fixed and enhancements added to the kernel for this release, refer to the kernel section of the Red Hat Enterprise Linux 6.4 Technical Notes.

Fibre Channel Protocol: End-To-End Data Consistency Checking

Data integrity between a host adapter and a storage server has been improved in Red Hat Enterprise Linux 6.4 by implementing the zFCP-specific part of the enhanced T10 DIF SCSI standard for End-To-End (E2E) data consistency checking.

Flash Express Support for IBM System z

Storage-Class Memory (SCM) for IBM System z is a class of data storage devices that combine properties of both storage and memory. SCM for System z now supports Flash Express memory. SCM increments can be accessed through Extended Asynchronous Data Mover (EADM) subchannels. Each increment is represented by a block device. This feature improves the paging rate and access performance for temporary storage, for example for data warehousing.

Open vSwitch Kernel Module

Red Hat Enterprise Linux 6.4 includes the Open vSwitch kernel module as an enabler for Red Hat's layered product offerings. Open vSwitch is supported only in conjunction with those products containing the accompanying user space utilities. Please note that without these required user space utilities, Open vSwitch will not function and can not be enabled for use. For more information, please refer to the following Knowledge Base article:

Oracle ASMLib Availability and Support

Oracle ASM (Automated Storage Management) is a data volume manager for Oracle databases. ASMLib is an optional utility that can be used on Linux systems to manage Oracle ASM devices. ASMLib consists of the following components:
  • kmod-oracleasm (open-source (GPL) kernel module package)
  • oracleasm-support (open-source (GPL) utilities package)
  • oracleasmlib (proprietary library package)
ASM features and functionality are available without ASMLib. The use of ASMLib does not affect database performance. The ASMLib kernel module package is now available in the Red Hat Enterprise Linux 6 Supplementary RHN channel. Red Hat Enterprise Linux 6 customers who use ASMLib can obtain the other two components using the instructions in the following KnowledgeBase article:
The ASMLib kernel module package is provided for the convenience of our customers via the "Supplementary” Red Hat Network (RHN) channel. Red Hat's support team will field ASMLib related calls and use commercially reasonable efforts to support the ASMLib kernel module until such efforts require knowledge of or modifications to Oracle's proprietary dependent component(s). That said, the ASMLib kernel module package ABIs are not guaranteed per Red Hat's Supplementary software package support terms available at:
Please note that Red Hat is continuing to develop fully open-source alternatives to ASMLib. Red Hat has provided a reference architecture for Oracle RAC clusters using upstream-accepted technologies such as dm-multipath and udev. This reference architecture is available at:

Comparison of Booted System and Dumped System

This feature allows you to compare a booted system with a dumped system to efficiently analyze changes that might be introduced by image migration. To identify a guest, stsi and stfle data is used. A new function, lgr_info_log() compares the current data (lgr_info_cur) with the last recorded one (lgr_info_last).

Perf Tool Updated

The perf tool has been updated to upstream version 3.6-rc7, which provides a large number of bug fixes and enhancements. The following is a list of notable enhancements:
  • Kprobe events support was added.
  • A new perf event command line syntax engine has been included, which allows curly brackets ({ and }) to be used for definition of event groups, for example: {cycles,cache-misses}.
  • The perf annotate browser has been enhanced to allow navigation through ASM calls and jumps.
  • The perf tool has been updated to provide a per-user view with the new --uid command line option. When used, perf shows tasks for a specified user only.
  • The perf tool now provides a wider variety of automated tests.

Uncore PMU Support

The kernel shipped with Red Hat Enterprise Linux 6.4 adds "uncore" Performance Monitoring Unit (PMU) support to the perf event subsystem for Intel Xeon Processor X55xx and Intel Xeon Processor X56xx family of processors. The "uncore" refers to subsystems in the physical processor package that are shared by multiple processor cores, for example the L3 cache. With uncore PMU support, performance data can be easily collected on a package level.
PMU events parsing has also been enabled to allow debugging via perf.

Reduced memcg Memory Overhead

Memory control groups maintain their own Least Recently Used (LRU) list to, for example, reclaim memory. This list was on top of the global per-zone LRU list. In Red Hat Enterprise Linux 6.4, the memory overhead for memcg was reduced by disabling the global per-zone LRU list and converting its users to operate on the per-memory cgroup lists instead.

Memory Reclaim and Compaction

The kernel shipped with Red Hat Enterprise Linux 6.4 uses reclaim and compaction for high-order allocation requests or under memory pressure.

Support of the Transactional Execution Facility and Runtime Instrumentation Facility

Support of the Transactional-Execution Facility (available with IBM zEnterprise EC12) in the Linux kernel helps eliminate software locking overhead that can impact performance and offer increased scalability and parallelism to drive higher transaction throughput. Support of the Runtime Instrumentation Facility (available with IBM zEnterprise EC12) provides an advanced mechanism to profile program code for improved analysis and optimization of the code generated by the new IBM JVM.

Fail-open Mode

Red Hat Enterprise Linux 6.4 adds support for a new fail-open mode when using netfilter's NFQUEUE target. This mode allows users to temporarily disable packet inspection and maintain connectivity under heavy network traffic.

kdump and kexec Kernel Dumping Mechanism for IBM System z Fully Supported

In Red Hat Enterprise Linux 6.4, the kdump/kexec kernel dumping mechanism is enabled for IBM System z systems as a fully supported feature, in addition to the IBM System z stand-alone and hypervisor dumping mechanism. The auto-reserve threshold is set at 4 GB; therefore, any IBM System z system with more than 4 GB of memory has the kdump/kexec mechanism enabled.
Sufficient memory must be available because kdump reserves approximately 128 MB by default. This is especially important when performing an upgrade to Red Hat Enterprise Linux 6.4. Sufficient disk space must also be available for storing the dump in case of a system crash.
You can configure or disable kdump through /etc/kdump.conf, system-config-kdump, or firstboot.

TSC Deadline Support for KVM

TSC deadline timer is a new mode in the Local APIC (LAPIC) timer, which generates one-shot timer interrupts based on the TSC deadline, in place of the current APIC clock count interval. It provides more precise timer interrupts (less than 1 tick) to benefit the OS scheduler. KVM now exposes this feature to guests.

Persistent Device Naming

This feature stores the mapping of device names (for example, sda, sdb, and others) and persistent device names (provided by udev in /dev/disk/by-*/) to kernel messages. This allows users to identify a device from kernel messages. The kernel /dev/kmsg log, which can be displayed with the dmesg command, now shows the messages for the symbolic links, which udev has created for kernel devices. These messages are displayed in the following format:
udev-alias: <device_name> (<symbolic_link> <symbolic link> …)
Any log analyzer can display these messages, which are also saved in /var/log/messages via syslog.

New linuxptp Package

The linuxptp package, included in Red Hat Enterprise Linux 6.4 as a Technology Preview, is an implementation of the Precision Time Protocol (PTP) according to IEEE standard 1588 for Linux. The dual design goals are to provide a robust implementation of the standard and to use the most relevant and modern Application Programming Interfaces (API) offered by the Linux kernel. Supporting legacy APIs and other platforms is not a goal.

Transparent Hugepages Documentation

Documentation for transparent hugepages has been added to the following file:

State of Support for Dump Targets

In Red Hat Enterprise Linux 6.4, the /usr/share/doc/kexec-tools-2.0.0/kexec-kdump-howto.txt file provides a comprehensive list of supported, unsupported, and unknown dump targets under section Dump Target support status.