Why is Yazi Fast?

[sxyazi]

This article assumes that you have already used Yazi and are familiar with most of its features.

Yazi has undergone significant optimizations to enhance user experience. It is designed entirely as an async program, handling all time-consuming tasks (I/O and CPU) as async tasks in a non-blocking, event-driven manner.

Tokio

Internally, Yazi uses Tokio as its async runtime: hold on! Tokio's async may not be "truly async" as you might perceive it!

Uh, okay. From an application-layer perspective, it indeed is async; however, from a system-level view, there are possibly better solutions.

But! This is not the current performance bottleneck for Yazi. Considering Yazi is a TUI app, unlike CLI programs like ls and eza that need to output all files immediately, Yazi has more optimization opportunities at the application-layer:

• For large directories (e.g., 100,000 files), Yazi uses chunked loading, which is unmatched by ls and eza since they must load everything at once.
• Yazi also preloads directory file lists in the background, an optimization that ls and eza do not possess.

I must express my gratitude to Tokio for providing an excellent and convenient way to realize these application-layer optimizations.

I believe that the benefits brought by these application-level optimizations are more noticeable compared to switching to solutions like io_uring. But I'm open to this and welcome any constructive PR.

Here is a relevant discussion on Reddit: https://www.reddit.com/r/rust/comments/16fxr58/comment/k066gmh/

Pre-Loading

Preloaders are part of Yazi's concurrent plugin system, and the entire pre-loading process is asynchronous and spans multiple threads. This means that preloaders can handle not only expensive IO tasks but also CPU-bound tasks! Here are some built-in preloaders in Yazi:

• Mimetype: The baseline. Yazi uses the file's mime-type as a reference for tasks such as opening, previewing, and style rendering, and internally utilizes file(1) to obtain the file's mime-type. For better performance, Yazi computes them for files of an entire page, rather than for each file individually, and the entire process is chunked to minimize response latency.
• Image: To accelerate image previews, Yazi uses a 2-pass process for image files. The first pass is preprocessing, which downscales the image based on user-set max_width/max_height and generates a compressed lossy image as a cache file, significantly reducing file size. The second pass occurs when the user actually switches to the file and downscales it again to fit the terminal size.
• Video: To speed up video previews, Yazi pre-converts them into images and goes through the first pass of image processing. When the user needs to display the video, it goes the same second pass.
• PDF: Similar to video.
• Directory size: Yazi lazily calculates the directory size only when the user sets sorting by file size, as it's a time-consuming operation.

Note: Except for size, all of these are paged, meaning that when you are on the first page, only the first few files will be pre-loaded.

For example, if your directory has 1000 files, your terminal height is 10, and you are on the second page, only files 11 to 20 will be processed. This greatly saves resources.

Discardable Tasks

Every preview task is discardable. When you navigate between files quickly and the previous file's triggered preview task is still not finished, it will be discarded directly, initiating a new task. This promotes resource utilization:

• For I/O tasks like loading directory lists, Tokio's abort is used;
• For CPU tasks like code highlighting, an Atomic is used to store a ticket, and it checks if the value changes on each line code highlight. If it changes, indicates that the current context has changed, and the entire highlighting task is discarded.
• For I/O and CPU tasks like previewer/preloader plugins, with Lua, Yazi can check whether these tasks are canceled when a specific number of CPU instructions. If canceled, it interrupts the execution of the Lua script immediately, avoiding wasting more I/O and CPU resources.

Code Highlighting

Yazi has built-in code highlighting and keeps it to a minimum for all text files: if your terminal height is 10, only the first 10 lines of the file are read and highlighted.

Other file managers that rely on external programs like bat need to wait for bat to finish highlighting the entire file before displaying only the first 10 lines.

In cases like JSON that require external program jq, Yazi kills jq directly after reading the first 10 lines to avoid unnecessary resource consumption.

Since code highlighting is a CPU-bound task, it is distributed among multiple blocking threads, managed through Tokio's spawn_blocking, and is also discardable.

Image Preview

Yazi not only has built-in code highlighting but also includes image decoding and downscaling - there is likely nothing faster than having it directly built-in. It is also distributed among multiple threads and is discardable.

Besides being fast, Yazi's built-in Kitty graphics protocol, Inline images protocol, and Sixel graphics format allow Yazi to finely control when to display or hide images.

This ensures that in Yazi, there won't be issues, like images stacking on top of each other, or image escape code breaking the entire screen, when navigating through images quickly, as stdout is locked while outputting these escape codes. This locking happens after all image data is prepared, so it has no impact on performance.

Yazi even supports partially erasing content in preview images, which is useful for pop-up components (Input, Select). The image won't overlap the input, and when the pop-up disappears, Yazi redraws the image to complete the erased portion automatically.

Async Task Scheduling

In Yazi, tasks are prioritized based on their severity automatically. Yazi categorizes tasks into two types:

• Macro tasks: Large and heavy tasks, such as copying large files, typically taking a long time to complete.
• Micro tasks: Small and urgent tasks, such as fetching file mime-type, pre-loading images, calculating directory size, and so on.

This is similar to having big and small cores in a CPU; when the big cores are idle, they help with the micro tasks. Yazi defaults to starting 5 micro workers and 10 macro workers, and these numbers can be configured by the user!

In addition, Yazi introduces a priority scheduling mechanism. Each task has 3 priority levels: low, normal, and high. High-priority tasks can preempt low-priority tasks, applying to both micro and macro tasks. This increases task concurrency, slowing down HOL blocking caused by queuing execution of sudden requests.

For complex tasks like file copying, a combination of micro and macro approaches is employed. Micro is used to gather a list of all files to be copied recursively, allowing users to see the number of tasks and their sizes in advance. Macro, on the other hand, handles the actual copying process.

The advantage of task scheduling extends beyond providing ample concurrency for I/O and CPU resources; it also indirectly mitigates the depletion of system resources (such as file handles and CPU) due to sudden task surges.

Other optimizations

The above optimizations are the most noticeable to users, but behind the scenes, Yazi has also done many other optimizations. Include but are not limited to:

• The re-implemented highly optimized natural sorting algorithm is ~6 times faster than the natord that eza uses in case-insensitive sorting.
• Yazi caches the directory state that has already been read, avoiding any unnecessary IO operations.
• When a file in a directory changes, it only updates the changed files rather than re-reading the entire directory list.
• Merges multiple renders triggered by multiple commands into a single render, avoiding unnecessary CPU consumption.
• Frequent updates to components, such as progress bars, are rendered independently, which is no cost compared to a complete render.
• The entire plugin system is designed with an asynchronous-first philosophy to avoid blocking the main thread with time-consuming tasks.

TODO

I'll find time to continue writing.

[matu3ba]

Excellent summary, although rough expected memory usage is not (fully) addressed yet (nit):

• 1. Downscaling factor or limits and number of images is afaiu, a sixel*width*height, but I do not understand how big videos are downscaled
• 2. chunked loading limits: prefetch is 2*height, leading to 3*heigh of terminal as the prefetched file with memory usage (if you do vim full page jumps), but the description is not accurate yet on what happens with huge files or what the memory cap is for prefetching
• 3. Behavior for unknown files (binary) etc is not described

[sxyazi]

Downscaling factor or limits and number of images is afaiu, a sixelwidthheight, but I do not understand how big videos are downscaled

Are you calculating real-time memory usage for image processing? This calculation method doesn't seem to be effective for resident memory. When Yazi displays an image, it encodes the image into a terminal-recognized image escape code and outputs it. After that, the image data is no longer stored in memory of Yazi, and the data is transferred to the terminal itself.

For videos, as mentioned in this README, use ffmpegthumbnailer to retrieve the cover and convert it to an image. It then goes through the same steps as with images.

chunked loading limits: prefetch is 2height, leading to 3heigh of terminal as the prefetched file with memory usage (if you do vim full page jumps), but the description is not accurate yet on what happens with huge files or what the memory cap is for prefetching

I don't quite understand this. How were 2*height and 3*height determined here?

Behavior for unknown files (binary) etc is not described

Seems I have explained it in the previous issue:

If you are talking about resident memory, for images, it is not applicable because they are reading cache from disk. For regular files, the MIME type of the file is pre-cached, and a BTreeMap<Url, String> is used to store the mapping between files and their MIME types.

[matu3ba]

How were 2height and 3height determined here?

------
above (shell height)
------
current (shell height) <<<<
------
below (shell height)
------

and potentially the same for the preview.

Thanks for clarifying. This all now makes sense to me.

[hakan-demirli]

I would like to share my experience with Yazi. It is definitely fast compared to lf on my x86 laptop thanks to bunch of async optimizations it offers. But, on a raspberry pi 4 over SSH it is laggy compared to lf.
I assume the latency of creating new threads or async calls in the background is too much for a raspi. It stutters quite often especially when I am surfing in linux root folders.

[sxyazi]

I assume the latency of creating new threads or async calls in the background is too much for a raspi

Yes, Yazi's default config is optimized for PCs, and on low-performance devices like the Raspberry Pi, a high concurrency setting can often have a counterproductive effect. You can try to reduce them to 1:

yazi/yazi-config/preset/yazi.toml

Lines 68 to 71 in 0ea9bcd

	[tasks]
	micro_workers = 5
	macro_workers = 10
	bizarre_retry = 5

I hadn't considered running Yazi on these devices before, hence added a limitation on minimum concurrency. I have now created a commit to remove this restriction.

Seems there are some other things we can do, such as disabling pre-caching for low-performance devices, would you like to file an issue for this?

[xaionaro]

Out of curiosity, how different yazi is from alacritty?

[sxyazi]

Moved to https://yazi-rs.github.io/blog/why-is-yazi-fast

[github-actions]

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