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Chapter 6. Kernel

Support for Ceph Block Devices

The libceph.ko and rbd.ko modules have been added to the Red Hat Enterprise Linux 7.1 kernel. These RBD kernel modules allow a Linux host to see a Ceph block device as a regular disk device entry which can be mounted to a directory and formatted with a standard file system, such as XFS or ext4.
Note that the CephFS module, ceph.ko, is currently not supported in Red Hat Enterprise Linux 7.1.

Concurrent Flash MCL Updates

Microcode level upgrades (MCL) are enabled in Red Hat Enterprise Linux 7.1 on the IBM System z architecture. These upgrades can be applied without impacting I/O operations to the flash storage media and notify users of the changed flash hardware service level.

Dynamic kernel Patching

Red Hat Enterprise Linux 7.1 introduces kpatch, a dynamic "kernel patching utility", as a Technology Preview. The kpatch utility allows users to manage a collection of binary kernel patches which can be used to dynamically patch the kernel without rebooting. Note that kpatch is supported to run only on AMD64 and Intel 64 architectures.

Crashkernel with More than 1 CPU

Red Hat Enterprise Linux 7.1 enables booting crashkernel with more than one CPU. This function is supported as a Technology Preview.

dm-era Target

Red Hat Enterprise Linux 7.1 introduces the dm-era device-mapper target as a Technology Preview. dm-era keeps track of which blocks were written within a user-defined period of time called an "era". Each era target instance maintains the current era as a monotonically increasing 32-bit counter. This target enables backup software to track which blocks have changed since the last backup. It also enables partial invalidation of the contents of a cache to restore cache coherency after rolling back to a vendor snapshot. The dm-era target is primarily expected to be paired with the dm-cache target.

Cisco VIC kernel Driver

The Cisco VIC Infiniband kernel driver has been added to Red Hat Enterprise Linux 7.1 as a Technology Preview. This driver allows the use of Remote Directory Memory Access (RDMA)-like semantics on proprietary Cisco architectures.

Enhanced Entropy Management in hwrng

The paravirtualized hardware RNG (hwrng) support for Linux guests via virtio-rng has been enhanced in Red Hat Enterprise Linux 7.1. Previously, the rngd daemon needed to be started inside the guest and directed to the guest kernel's entropy pool. Since Red Hat Enterprise Linux 7.1, the manual step has been removed. A new khwrngd thread fetches entropy from the virtio-rng device if the guest entropy falls below a specific level. Making this process transparent helps all Red Hat Enterprise Linux guests in utilizing the improved security benefits of having the paravirtualized hardware RNG provided by KVM hosts.

Scheduler Load-Balancing Performance Improvement

Previously, the scheduler load-balancing code balanced for all idle CPUs. In Red Hat Enterprise Linux 7.1, idle load balancing on behalf of an idle CPU is done only when the CPU is due for load balancing. This new behavior reduces the load-balancing rate on non-idle CPUs and therefore the amount of unnecessary work done by the scheduler, which improves its performance.

Improved newidle Balance in Scheduler

The behavior of the scheduler has been modified to stop searching for tasks in the newidle balance code if there are runnable tasks, which leads to better performance.

HugeTLB Supports Per-Node 1GB Huge Page Allocation

Red Hat Enterprise Linux 7.1 has added support for gigantic page allocation at runtime, which allows the user of 1GB hugetlbfs to specify which Non-Uniform Memory Access (NUMA) Node the 1GB should be allocated on during runtime.

New MCS-based Locking Mechanism

Red Hat Enterprise Linux 7.1 introduces a new locking mechanism, MCS locks. This new locking mechanism significantly reduces spinlock overhead in large systems, which makes spinlocks generally more efficient in Red Hat Enterprise Linux 7.1.

Process Stack Size Increased from 8KB to 16KB

Since Red Hat Enterprise Linux 7.1, the kernel process stack size has been increased from 8KB to 16KB to help large processes that use stack space.

uprobe and uretprobe Features Enabled in perf and systemtap

In Red Hat Enterprise Linux 7.1, the uprobe and uretprobe features work correctly with the perf command and the systemtap script.

End-To-End Data Consistency Checking

End-To-End data consistency checking on IBM System z is fully supported in Red Hat Enterprise Linux 7.1. This enhances data integrity and more effectively prevents data corruption as well as data loss.

DRBG on 32-Bit Systems

In Red Hat Enterprise Linux 7.1, the deterministic random bit generator (DRBG) has been updated to work on 32-bit systems.

NFSoRDMA Available

As a Technology Preview, the NFSoRDMA service has been enabled for Red Hat Enterprise Linux 7.1. This makes the svcrdma module available for users who intend to use Remote Direct Memory Access (RDMA) transport with the Red Hat Enterprise Linux 7 NFS server.

Support for Large Crashkernel Sizes

The Kdump kernel crash dumping mechanism on systems with large memory, that is up to the Red Hat Enterprise Linux 7.1 maximum memory supported limit of 6TB, has become fully supported in Red Hat Enterprise Linux 7.1.

Kdump Supported on Secure Boot Machines

With Red Hat Enterprise Linux 7.1, the Kdump crash dumping mechanism is supported on machines with enabled Secure Boot.

Firmware-assisted Crash Dumping

Red Hat Enterprise Linux 7.1 introduces support for firmware-assisted dump (fadump), which provides an alternative crash dumping tool to kdump. The firmware-assisted feature provides a mechanism to release the reserved dump memory for general use once the crash dump is saved to the disk. This avoids the need to reboot the system after performing the dump, and thus reduces the system downtime. In addition, fadump uses of the kdump infrastructure already present in the user space, and works seamlessly with the existing kdump init scripts.

Runtime Instrumentation for IBM System z

As a Technology Preview, support for the Runtime Instrumentation feature has been added for Red Hat Enterprise Linux 7.1 on IBM System z. Runtime Instrumentation enables advanced analysis and execution for a number of user-space applications available with the IBM zEnterprise EC12 system.

Cisco usNIC Driver

Cisco Unified Communication Manager (UCM) servers have an optional feature to provide a Cisco proprietary User Space Network Interface Controller (usNIC), which allows performing Remote Direct Memory Access (RDMA)-like operations for user-space applications. As a Technology Preview, Red Hat Enterprise Linux 7.1 includes the libusnic_verbs driver, which makes it possible to use usNIC devices via standard InfiniBand RDMA programming based on the Verbs API.

Intel Ethernet Server Adapter X710/XL710 Driver Update

The i40e and i40evf kernel drivers have been updated to their latest upstream versions. These updated drivers are included as a Technology Preview in Red Hat Enterprise Linux 7.1.