Linux recently introduced two small patches — one just a single line long and the other around 20 lines. Despite their size, these patches have had an unexpectedly significant impact on the performance of the kernel, the core of this renowned free operating system.

A recent patch known as ‘mmap’ has delivered a staggering performance boost to the Linux kernel. According to Intel’s automated Linux kernel testing bot, this single line of code led to an incredible 3888.9% improvement — yes, nearly 4,000%!

The patch tweaks how certain blocks of memory, called THPs (Transparent Huge Pages), are aligned. These large memory blocks are designed to enhance performance by reducing the overhead of managing smaller chunks.

Previously, the kernel strictly enforced a specific alignment rule for larger memory allocations. While the intention was good, this approach caused issues for some programs that couldn’t work well with the rule, leading to significant slowdowns. The ‘mmap’ patch solves this problem by introducing a more flexible alignment method.

How the Patch Resolves Memory Fragmentation Issues

The ‘mmap’ patch addresses a performance issue that arose after a kernel change over a year ago. This problem became evident in the ‘cactusBSSN’ benchmark, which started running up to 600% slower on some systems.

The slowdown occurred because this program uses memory in unconventional sizes, such as 4632 kB. Before the kernel change, these sizes could be grouped into large, manageable blocks. However, the update caused these blocks to fragment into smaller pieces, leaving gaps of unusable “empty space” in memory.

This fragmentation made it harder for the system to locate and utilize memory efficiently. The issue didn’t stop with the benchmark; it also affected real-world applications like Darktable, a widely used photo editing program. The patch resolves this by restoring efficient memory handling for such cases.

Torvalds’ 21-Line Patch Enhances Kernel Performance

Linus Torvalds, the creator of the Linux kernel, has once again showcased his expertise and leadership in its development. In one of his recent contributions — a patch just 21 lines long — Torvalds managed to boost the Linux kernel’s multi-threaded performance by 2.6%.

This improvement has already been incorporated into the stable release of the Linux kernel, starting with version 6.12, which was launched less than a month ago.

Balancing Security and Performance in the Kernel

The patch modifies the Linux kernel’s copy_from_user() function, primarily removing the barrier_nospec() security measure. This measure was initially introduced to mitigate the Meltdown and Spectre vulnerabilities.

Discovered in 2018, these vulnerabilities exploited speculative execution, a feature in modern processors that predicts and preloads instructions to boost performance.

While barrier_nospec() was effective in addressing these security concerns, it had a significant downside — it severely slowed down performance. The impact was particularly noticeable in multi-threaded systems and applications with high call frequencies, such as proxy servers and databases. The patch addresses this tradeoff by eliminating the measure, improving system efficiency.

Torvalds’ Approach: Optimizing Security without Compromising Speed

The challenge of balancing security with performance has been a key issue in the evolution of the Linux kernel. Linus Torvalds, known for his focus on speed and efficiency, has found a solution that addresses both concerns effectively.

Staying true to his philosophy, Torvalds introduced an optimization using pointer masking. In this approach, an invalid user-provided address is replaced with an address consisting of all ones (‘111…111’). This not only maintains necessary security measures but also eliminates unnecessary operations, enhancing system efficiency.

Automated tests in multi-threaded environments confirmed the positive impact of this change, showing a 2.6% improvement in performance. While this may seem small, the effect becomes much more significant when considering the widespread use of Linux, from servers and cloud services to local devices.