Ingo Molnár 提出讓 Linux Kernel 編譯速度提昇的 mega patch

Hacker News 首頁上看到「Massive ~2.3k Patch Series Would Improve Linux Build Times 50~80% & Fix "Dependency Hell"」這個,對應到 mailing list 上的信件是「* [PATCH 0000/2297] [ANNOUNCE, RFC] "Fast Kernel Headers" Tree -v1: Eliminate the Linux kernel's "Dependency Hell"」這個,看到「0000/2297」這個 prefix XDDD

他主要是想要改善 Linux Kernel 的 compile 時間 (從 project 的名稱「Fast Kernel Headers」可以看到),只是沒想到會縮短這麼多。另外一方面也順便處理了 dependency hell 的問題 (改善維護性)。

測試出來的結果相當驚人,從 231.34 +- 0.60 secs (15.5 builds/hour) 到 129.97 +- 0.51 secs (27.7 builds/hour),以編譯次數來看的話是 78% 的改善。如果以 CPU time 來看的話,從 11,474,982.05 msec cpu-clock 降到 7,100,730.37 msec cpu-clock,也是以編譯次數來算的話,有 61.6% 的改善...

這是花了一年多的時間嘗試才達成的目標,嘗試不同的方法,前幾次雖然都有改善,但改善幅度太小,變動卻太大,他覺得不值得丟出來,直到第三次才達成這樣的目標...

第一次:

When I started this project, late 2020, I expected there to be maybe 50-100 patches. I did a few crude measurements that suggested that about 20% build speed improvement could be gained by reducing header dependencies, without having a substantial runtime effect on the kernel. Seemed substantial enough to justify 50-100 commits.

第二次:

But as the number of patches increased, I saw only limited performance increases. By mid-2021 I got to over 500 commits in this tree and had to throw away my second attempt (!), the first two approaches simply didn't scale, weren't maintainable and barely offered a 4% build speedup, not worth the churn of 500 patches and not worth even announcing.

第三次:

With the third attempt I introduced the per_task() machinery which brought the necessary flexibility to reduce dependencies drastically, and it was a type-clean approach that improved maintainability. But even at 1,000 commits I barely got to a 10% build speed improvement. Again this was not something I felt comfortable pushing upstream, or even announcing. :-/

然後基於第三次的成果覺得有望,意外的發現後續的速度改善比想像中的多非常多:

But the numbers were pretty clear: 20% performance gains were very much possible. So I kept developing this tree, and most of the speedups started arriving after over 1,500 commits, in the fall of 2021. I was very surprised when it went beyond 20% speedup and more, then arrived at the current 78% with my reference config. There's a clear super-linear improvement property of kernel build overhead, once the number of dependencies is reduced to the bare minimum.

這次的 patch 雖然超大包,但看起來對於 compile 時間改善非常多,應該會有不少討論... 消息還蠻新的 (台灣時間今天早上五點的信),晚點可以看一下其他大老出來回什麼...

JavaScript Framework 不可避免的成本

看到「The Baseline Costs of JavaScript Frameworks」這篇文章在研究目前主流 JavaScript Framework 無法避免的成本到底有多高。

文章的結論是目前常見的 JavaScript Framework 其實都很肥重,在網路速度不快的地方得花不少時間下載,在非旗艦的手機上會需要花不少時間處理 (parse & compile)。

這是 gzip 後的大小:

這是 parse & compile 的時間:

這是下載時間 (扣除 latency 與 TLS connection 建立時間):

並不是說不能用,但重點會在客群:

But it’s important to consider your audience. If you’re building for resource constrained devices — which you certainly are if your product targets a country like India — you could consider using a lighter framework such as Riot or Preact. Your users will thank you.

最後有建議如果只是要呈現資訊,不要用整套 JavaScript Framework,在有需要互動的地方另外寫就好了:

For websites that primarily display content, it’s more efficient and cost-effective to just send some server-rendered HTML down the wire. If there are areas of your website that require interactivity, you can always use JavaScript to build those specific parts.

V8 version 6.5 (Chrome 65) 的改變

V8 version 6.5 將會有不少改變:「V8 release v6.5」。

其中因為 Spectre 的關係,新的 V8 設計了 Untrusted code mode,拿來跑不信任的程式,裡面會設計反制措施。而且這在新版的 Chrome 將會預設開啟:

In response to the latest speculative side-channel attack called Spectre, V8 introduced an untrusted code mode. If you embed V8, consider leveraging this mode in case your application processes user-generated, not-trustworthy code. Please note that the mode is enabled by default, including in Chrome.

另外是針對 WebAssembly 提供邊下載邊 compile 的能力,這讓速度大幅提昇。在原文是拿一個比較大包的 WebAssembly 來測試:

For the graph below we measure the time it takes to download and compile a WebAssembly module with 67 MB and about 190,000 functions. We do the measurements with 25 Mbit/sec, 50 Mbit/sec, and 100 Mbit/sec download speed.

可以看到網路不夠快的使用者就會直接被 compile 速度跟上,讓瀏覽器在下載時就做一些事情。

另外在某些情況下對 Array 的操作會有大幅改善:

這些新功能與改善都會在 Chrome 65 推出。依照「Chrome Platform Status」這邊的資料,stable 版預定在三月初,beta 版應該是要出了... (雖然上面寫著 2/1,但目前好像還沒更新)