This topic describes the kernel features and interfaces supported by Alibaba Cloud Linux 2. This topic is intended for users who are familiar with the Linux kernel and want to use Linux kernel features.

Memory

References Description
Memcg Exstat feature Alibaba Cloud Linux 2 provides the memcg exstat (Extend/Extra) feature in kernel versions 4.19.91-18.al7 and later.
Memcg global minimum watermark rating Alibaba Cloud Linux 2 provides the memcg global minimum watermark rating feature. The global minimum watermark of resource-consuming tasks is increased to trigger direct memory reclaim. The global minimum watermark of latency-sensitive tasks is decreased to prevent direct memory reclaim. When a resource-consuming task requests a large amount of memory, the global minimum watermark is increased to throttle resources used for the task for a short period to prevent direct memory reclaim for latency-sensitive tasks. After a specified amount of memory is reclaimed by means of global kswapd backend reclaim, the resource-consuming task is no longer throttled.
Memcg QoS feature of the cgroup v1 interface The memcg quality of service (QoS) can be used to control locks and limits on memory usage in a memcg. Alibaba Cloud Linux 2 provides the memcg QoS feature supported by the cgroup v1 interface in kernel versions 4.19.91-18.al7 and later.
Memcg backend asynchronous reclaim Alibaba Cloud Linux 2 provides the backend asynchronous reclaim feature for memcgs. This feature differs from the global kswapd kernel thread because it uses the workqueue mechanism instead of a corresponding memcg kswapd kernel thread.
THP reclaim Alibaba Cloud Linux 2 provides the Transparent Huge Pages (THP) reclaim feature in kernel versions 4.19.91-24.al7 and later. This feature is used to address memory usage issues caused by THP, such as out of memory (OOM) errors.

Network

References Description
TCP-RT configurations Alibaba Cloud Linux 2 provides the TCP-layer service monitoring (TCP-RT) feature in kernel versions 4.19.91-21.al7 and later.
Change the TCP TIME-WAIT timeout period In normal Linux kernels, TCP/IP connections remain in the TIME-WAIT state for 60 seconds. The length of this period cannot be changed. However, in specific scenarios such as heavy TCP loads, network performance can be improved if this period is shortened. To resolve this issue, Alibaba Cloud Linux 2 provides a kernel interface in kernel versions 4.19.43-13.al7 and later to change the period of time that TCP/IP connections remain in the TIME-WAIT state before the system closes them.

Storage

References Description
Enable the cgroup writeback feature Alibaba Cloud Linux 2 provides the cgroup writeback feature for the cgroup v1 kernel interface in kernel versions 4.19.36-12.al7 and later. This feature allows you to limit the buffered I/O rate when you use the cgroup v1 kernel interface.
Configure the weight-based throttling feature of blk-iocost Alibaba Cloud Linux 2 provides the weight-based throttling feature (blk-iocost) based on the cost model in kernel versions 4.19.81-17.al7.x86_64 and later. This feature improves the weight-based disk throttling feature of the I/O subsystem (blkcg) within a kernel.
Enhance the monitoring of block I/O throttling Alibaba Cloud Linux 2 provides interfaces in kernel versions 4.19.81-17.al7 and later to better monitor Linux block I/O throttling.
Use the JBD2 optimization interface JBD2 is the kernel thread of ext4 file systems. JBD2 often experiences the shadow (BH_Shadow) state when it is being used, which can affect the system performance. To resolve this issue, Alibaba Cloud Linux 2 provides an interface to optimize JBD2 in kernel versions 4.19.81-17.al7 and later.
Create a hard link across project quotas By default, ext4 file systems have constraints that disallow hard links to be created across project quotas. However, in practice, some scenarios require hard links be created. Alibaba Cloud Linux 2 provides a custom interface to bypass the constraints of ext4 file systems and create hard links across project quotas.
Track I/O latency Alibaba Cloud Linux 2 optimizes the /proc/diskstats interface that provides raw data for the I/O latency analysis tool iostat. Alibaba Cloud Linux 2 can calculate the durations of read, write, and special I/O (discard) operations on the device side. In addition, Alibaba Cloud Linux 2 provides the bcc tool to track I/O latency.
Detect I/O hangs of file systems and block layers An I/O hang occurs when the system becomes unstable or even goes down due to time-consuming I/O requests. Alibaba Cloud Linux 2 extends the core data structure and provides the feature to identify and detect I/O hangs at low system overheads.

Monitoring

References Description
Enable the PSI feature for cgroup v1 Alibaba Cloud Linux 2 provides the Pressure Stall Information (PSI) feature for the cgroup v1 interface in kernel versions 4.19.81-17.al7 and later. This feature allows you to monitor CPUs, memory, and I/O performance.

Scheduling

References Description
Enable the CPU burst feature for cgroup v1 Alibaba Cloud Linux 2 provides the CPU burst feature for the cgroup v1 interface in kernel versions 4.19.91-22.al7 and later. The CPU burst feature allows CPU-throttled containers to burst their CPU utilization for higher performance and lower latency.
Group identity feature Alibaba Cloud Linux 2 provides the group identity feature. This feature allows you to configure different identities for CPU cgroups to prioritize process tasks in the cgroups.

Others

References Description
Enhance the visibility of container resources Alibaba Cloud Linux 2 provides kernel interfaces for the container resource visualization feature to enhance the visibility of container resources.