[UnrealEngine][讀書心得] 關於UnrealEngine中的ISPC與SIMD的一些雜談

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SIMD(Single Instruction Multiple Data)指的是用一個指令同時處理多個資料這個概念,只是我們常常被誤導說,在CPU上可以使用SIMD指令來加速──這種說法其實並不精確。對,我就是被誤導的那個。在UE4.23時官方整合了ISPC,以更好的幫助CPU做平行化加速,我之前一直把這東西跟SIMD畫上等號,沒去仔細思考這東西的存在到底想解決什麼問題。

根據指令數跟跟資料的處理方式,這個概念其實是根據Flynn’s Taxonomy來進行分類,而SIMD只是其中一種。

SIMD就現行的計算機架構上,分為以下三種:

  1. Vector Architecture:這個架構在以前是是超級電腦的代名詞,最常在教科書上提到的是1975年的Cray-1,第一個使用這個架構的計算機。
  2. Multimedia SIMD Instruction set extension:看名字就可以知道,它的目的是針對多媒體應用的ISA的擴充指令集,例如圖像、影片等等影音需求。基本上是架構在Scalar Processor之上的擴充。
  3. GPU:這個比較好理解,他在架構上有點像是Vector Architecture,但有些微妙的不同。

不過由於Vector Architecture在商業競爭上輸給了Intel因此逐漸式微,以Scalar Processor為主的設計主宰了過去將近30~40年的時間。輸掉的原因有很多,但主要並不是技術問題,而是那個時代大部份的人不需要那麼強的計算力,客戶太少,再加上摩爾定律的推波助瀾下,人們的注意力自然就只集中在由Intel主推擁有最多客戶的架構上。然而近來由於AI時代的來臨,Vector Architecture架構又重新回到了人們眼界中了,例如google tensor裡面的TPU就包含了Vector processor,以及最近很火紅的RISC-V,由於設想目標是embedded devices,不像個人電腦需要考慮兼容性,因此可以大膽的拋棄舊包袱,轉而使用相關的Vector Operation指令。

另一方面,關於Multimedia SIMD,主要是由於Intel在網路起飛的時代發現我們基於Scalar Processor的架構還是需要Vector Architecture那種一個指令同時處理多個資料的加速能力,因此他的解決方式非常粗暴,就是加入新的register跟指令集的方式來支援這個概念。不過當我們開始疊加register跟指令集之後就開始失控了,每我們加一組新的register,我們就要追加一堆新的指令集來支援相關Vector操作,從XMM(128 bit)、YMM(256 bit)到ZMM(512 bit)。這些指令集的擴充非常快速,從1996年開始,幾乎每2~3年就有新的特性加入,然而為了兼容性,新的指令集在加入x86系統後就非常難被移除,因此現代指令集暴增的這麼快速有很大的原因可以歸究於Multimedia SIMD的發展。

接下來讓我們來稍微看一下這些擴充指令集,感受一下它發展的混亂吧:從1996年的MMX,它總共有57條擴充指令使用,到SSE又追加了70條,接著SSE2加入了144條指令,而SSE3只是在原有架構下的擴充,所以只追加了13條指令。接下來則是SSE4.1跟4.2,又分別加入了47條跟7條。前面的架構使用到的是XMM register,接下來為了增加平行化的數量,Intel推出了AVX系列,使用的是YMM register,然後又更進一步的推出了AVX-512,使用ZMM register。可惜的是我目前沒找到AVX的指令集數量,不過我猜應該有上百個。問了一下AI,他是跟我說要看所使用的具體版本。

對於最新的AVX512,Linus Torvalds在2020年的時曾經破口大罵說:我希望AVX-512可以死的痛苦一點(I hope AVX512 dies a painful death)。希望intel不要靠增加魔法指令來讓跑分好看,要他們回到正軌。

基本上這玩意耗能大,大部份的人享受不到好處,又不是什麼科研計算,也不是server,一般人在使用電腦的時候又常常需要切換process執行,這代表作業系統會不斷的進行Context Switch,要知道,每次Context Switch是需要把當前計算用到的register保存下來,在這一讀一寫之間很容易讓使用者感受到延遲,而且還多佔了空間。

另外在Intel 12th Gen CPU他設計了二種不同架構的P跟E Core,這是常見的大小核架構,只是小核不支援AVX- 512,沒辦法相容的結果Intel就乾脆把這東西關掉了,雖然能修改BIOS把這東西開回來,不過看起來有點麻煩。這玩意目前看起來確實是半殘了,今年的13代一樣延續同樣的架構因此不支援AVX512,不知道這指令會不會如Linus所期望的痛苦的死去,就讓我們看下去。不過令人意外的是AMD Zen 4之後的CPU倒是支援這套指令集的。

對於我們程式設計師而言,非常痛苦的是,每次有追加新的指令集我們都需要手動去include相關的header檔,然後額外針對新的指令撰寫新的程式。

見: https://stackoverflow.com/questions/11228855/header-files-for-x86-simd-intrinsics

有了以上知識,我們回頭來看Intel ISPC就可很清楚的掌握這技術到底想解決什麼問題了。在官網上這技術的定義是SPMD(single program, multiple data),就是說我們只要像GPU寫Shader Code一樣,把部份C++ code用ispc專用的script改寫成一段「program」存成.ispc這個檔案,在編譯階段它就會自動幫我們把這個檔案拉進去轉成目標指令集的樣子,這樣子的話,當有新的指令集被加入後,我們只需要重新編譯程式就行,而不需要再去改寫程式。當然,還要重新編譯新程式這點還是蠻煩人的,不過這個就是擴充指令集的原罪。

根據Epic官方在Siggraph2019的演講,目前ISPC主要是用在Animation跟Chaos這二個系統上,在部份關鍵的程式碼上大約會有3倍左右的性能提升。不過需要注意的是,在使用ISPC後,那些程式片段沒辦法享受到PGO(Profile-Guided Optimization)帶來的效能提升。

見:Causing Chaos: Physics and Destruction in UE4 | SIGGRAPH 2019 | Unreal Engine

參考:

Linux之父怒罵Intel AVX-512:「我希望它死得痛苦點...」 https://news.xfastest.com/others/82781/linus-torvalds-i-hope-intels-avx-512-dies-a-painful-death/

硬科技:淺談x86的SIMD指令擴張史(上):MMX到SSE  https://www.cool3c.com/article/152918

RISC-V Vector Instructions vs ARM and x86 SIMD  https://medium.com/swlh/risc-v-vector-instructions-vs-arm-and-x86-simd-8c9b17963a31

從 Pentium 回顧 x86 處理器到底哪裡難做 https://blog.moneydj.com/news/2020/10/02/%E5%BE%9E-pentium-%E5%9B%9E%E9%A1%A7-x86-%E8%99%95%E7%90%86%E5%99%A8%E5%88%B0%E5%BA%95%E5%93%AA%E8%A3%A1%E9%9B%A3%E5%81%9A/

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