How ByteDance Cut Kexec Reboot Time by 500ms with Linux Kernel Tweaks

ByteDance has been actively optimizing the Linux kernel in recent years, and its latest contribution focuses on speeding up kernel reboots using Kexec.

As their products such as Douyin and its overseas versions rely on large numbers of Linux servers, ByteDance engineers aim to shave off a few milliseconds from server start/reboot times to reduce downtime.

Like many large organizations, ByteDance uses Kexec to migrate to new kernels for security, maintenance, and performance reasons, avoiding full POST cycles. However, even with Kexec, the bootloader can still cause noticeable pause.

The newly contributed “faster kexec reboot” patch series, as the name suggests, accelerates Kexec reboot speed, targeting a reduction of about 500 ms, bringing the time to launch a kernel via Kexec down to roughly 15 ms.

Key optimizations include enabling uncompressed kernels on x86/x86_64 to avoid decompression and memory copying, and reusing memory retained by a crashed kernel for normal Kexec operations. Tests show that enabling an uncompressed kernel on x86 alone can cut boot time by over 150 ms, though the kernel image size grows from 8.5 MiB to 53 MiB.

The patch series touches roughly 100 lines of Linux kernel code, and ByteDance hopes to upstream these changes to achieve a half‑second reduction in Kexec reboot latency.

Related link: https://lore.kernel.org/lkml/[email protected]/