This topic summarizes the features and kernel interfaces supported by Alibaba Cloud Linux 2.

Use Alibaba Cloud Linux 2 images

The following table describes the operations on Alibaba Cloud Linux 2 images.

Documentation Description
Use Alibaba Cloud Linux 2 images in an on-premises environment Alibaba Cloud Linux 2 images can be downloaded to and used on local kernel-based virtual machines (KVMs).
Use YUM to perform security updates You can use YUM to query, check, and install security updates for Alibaba Cloud Linux 2.

Function interfaces supported by Alibaba Cloud Linux 2

If you have knowledge about the Linux kernel and need to use Linux kernel features, you can perform operations based on the Alibaba Cloud Linux 2 kernel features and interfaces described in the following table.

Documentation Description
Enable the cgroup writeback feature Alibaba Cloud Linux 2 supports the cgroup writeback feature for the cgroup v1 kernel interface in the 4.19.36-12.al7 kernel version or 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 the 4.19.81-17.al7.x86_64 kernel version or later. This feature improves the weight-based disk throttling feature of the I/O subsystem (blkcg) within a kernel.
Enable the PSI feature for the cgroup v1 interface Alibaba Cloud Linux 2 supports the Pressure Stall Information (PSI) feature for the cgroup v1 interface in the 4.19.81-17.al7 kernel version or later. This feature allows you to monitor the CPU, memory, and I/O performance.
Change the TCP TIME-WAIT timeout period In the Linux kernel, TCP/IP connections remain in the TIME-WAIT state for 60 seconds. The length of this period cannot be changed. However, in some scenarios such as heavy TCP loads, network performance can be improved if this period is reduced. In this context, Alibaba Cloud Linux 2 provides a kernel interface in the 4.19.43-13.al7 kernel version or later to change the length of the TCP TIME-WAIT timeout period.
Enhance the monitoring of block I/O throttling Alibaba Cloud Linux 2 provides interfaces in the 4.19.81-17.al7 kernel version or later to better monitor Linux block I/O throttling.
Use the JBD2 optimization interface JBD2 is the kernel thread of the ext4 file system. It often experiences the shadow (BH_Shadow) state during its use, which can affect the system performance. To solve this problem, Alibaba Cloud Linux 2 provides an interface in the 4.19.81-17.al7 kernel version or later to optimize JBD2.
Create a hard link across project quotas By default, the ext4 file system contains constraints. You are not allowed to create hard links across project quotas. However, in practice some scenarios require the creation of hard links. Alibaba Cloud Linux 2 provides a custom interface that can bypass the constraints of the ext4 file system to create hard links across project quotas.
Track I/O latency Alibaba Cloud Linux 2 optimizes the /proc/diskstats interface, which is the raw data source of the I/O latency analysis tool iostat. Alibaba Cloud Linux 2 can calculate the duration of time in read, write and special I/O (discard) on the device. 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. To accurately detect I/O hangs, Alibaba Cloud Linux 2 extends the core data structure and provides the feature to locate and detect I/O hangs with low system overheads.
Memcg global minimum watermark rating Alibaba Cloud Linux 2 provides the memcg global minimum watermark rating feature. The global wmark_min of resource-consuming tasks is increased to trigger direct memory reclaim. The global wmark_min of latency-sensitive tasks is decreased to avoid direct memory reclaim. This way, when a resource-consuming task requests a large amount of memory, an increase in the global wmark_min throttles the resources used for the task for a short period to avoid direct memory reclaim for latency-sensitive tasks. After a certain amount of memory is reclaimed through global kswapd backend reclaim, the resource-consuming task is no longer throttled.
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 in that it uses the workqueue mechanism instead of a corresponding memcg kswapd kernel thread.
Memcg QoS feature of the cgroup v1 interface The memory control group (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 in the 4.19.91-18.al7 kernel version or later.
Memcg Exstat feature Alibaba Cloud Linux 2 provides the memcg exstat (Extend/Extra) feature in the 4.19.91-18.al7 kernel version or later.