2010年3月31日 星期三

什麼是QuickPath Interconnect


  Intel的QuickPath Interconnect技術縮寫為QPI,譯為快速通道互聯。事實上它的官方名字叫做CSI,Common System Interface公共系統界面,用來實現芯片之間的直接互聯,而不是在通過FSB連接到北橋,矛頭直指AMD的HT總線。無論是速度、帶寬、每個針腳的帶寬、功耗等一切規格都要超越HT總線。

QPI的技術特點

  帶寬更大
  Intel的QuickPath Interconnect技術縮寫為QPI,譯為快速通道互聯。事實上它的官方名字叫做CSI,Common System Interface公共系統界面,用來實現芯片之間的直接互聯,而不是在通過FSB連接到北橋,矛頭直指AMD的HT總線。無論是速度、帶寬、每個針腳的帶寬、功耗等一切規格都要超越HT總線。
  QPI最大的改進是採用單條點對點模式下,QPI的輸出傳輸能力非常驚人,在4.8至6.4GT/s之間。一個連接的每個方向的位寬可以是5、10、20bit。因此每一個方向的QPI全寬度鏈接可以提供12至16BG/s的帶寬,那麼每一個QPI鏈接的帶寬為24至32GB/s。
  效率更高
  此外,QPI另一個亮點就是支持多條系統總線連接,Intel稱之為multi-FSB。系統總線將會被分成多條連接,並且頻率不再是單一固定的,也無須如以前那樣還要再經過FSB進行連接。根據系統各個子系統對數據吞吐量的需求,每條系統總線連接的速度也可不同,這種特性無疑要比AMD目前的Hypertransport總線更具彈性。

QPI與FSB的區別

  FSB正離我們遠去
  
眾所周之,前端總線(Front Side Bus,簡稱FSB)是將CPU中央處理器連接到北橋芯片的系統總線,它是CPU和外界交換數據的主要通道。前端總線的數據傳輸能力對計算機整體性能影響很大,如果沒有足夠帶寬的前端總線,即使配備再強勁的CPU,用戶也不會感覺到計算機整體速度的明顯提升。
  目前intel處理器主流的前端總線頻率有800MHz、1066MHz、1333MHz幾種,而就在2007年11月,intel再度將處理器的前端總線頻率提升至1600MHz(默認外頻400MHz),這比2003年最高的800MHzFSB總線頻率整整提升了一倍。這樣高的前端總線頻率,其帶寬多大呢?前端總線為1333MHz時,處理器與北橋之間的帶寬為10.67GB/s,而提升到1600MHz能達到12.8GB/s,增加了20%。
  雖然intel處理器的前端總線頻率看起來已經很高,但與同時不斷提升的內存頻率、高性能顯卡(特別是雙或多顯卡系統)相比,CPU與芯片組存在的前端總線瓶頸仍未根本改變。例如1333MHz的FSB所提供的內存帶寬是1333MHz×64bit/8=10667MB/s=10.67GB/s,與雙通道的DDR2-667內存剛好匹配,但如果使用雙通道的DDR2-800、DDR2-1066的內存,這時FSB的帶寬就小於內存的帶寬。更不用說和未來的三通道和更高頻率的DDR3內存搭配了(Nehalem平台三通道DDR3-1333內存的帶寬可達32GB/s)。
  與AMD的HyperTransport(HT)總線技術相比,FSB的帶寬瓶頸也很明顯。HT作為AMD CPU上廣為應用的一種端到端的總線技術,它可在內存控制器、磁盤控制器以及PCI-E總線控制器之間提供更高的數據傳輸帶寬。HT1.0在雙向32bit模式的總線帶寬為12.8GB/s,其帶寬便可匹敵目前最新的FSB帶寬。2004年AMD推出的HT2.0規格,最大帶寬又由1.0的12.8GB/s提升到了22.4GB/s。而最新的HT3.0又將工作頻率從HT2.0最高的1.4GHz提高到了2.6GHz,提升幅度幾乎又達到了一倍。這樣,HT3.0在2.6GHz高頻率32bit高位寬運行模式下,即可提供高達41.6GB/s的總線帶寬(即使在16bit的位寬下也能提供20.8GB/s 帶寬),相比FSB優勢明顯,應付未來兩年內內存、顯卡和處理器的升級需要也沒有問題。
  面對這種帶寬上的劣勢,雖然intel通過對市場的準確把握,以及其他優勢技術上的彌補(如指令集優勢、如CPU效率上intel的酷睿2雙核共享二級緩存互聯架構要明顯優於AMD HT互聯下的的雙核架構等等),讓AMD的帶寬優勢並沒有因此轉化為勝勢,但intel要想改變這種處理器和北橋設備之間帶寬捉襟見肘的情況,縱使在現可在技術上將FSB頻率進一步提高到2133MHz,也難以應付未來DDR3內存及多顯卡系統所帶來的帶寬需求。Intel推出新的總線技術勢在必行。
  當世界失去FSB我們還有QPI
  
Intel自身也清醒的認識到,要想在通過單純提高處理器的外頻和FSB,也難以像以前那樣帶來更好的性能提升。採用全新的Nehalem架構的intel下一代CPU讓我們看到了英特爾變革的決心。目前已經正式發佈,基於該架構的代號為Boomfield第一款處理器,我們可以看見很多很多技術的細節——該處理器擁有全新的規格和性能,採用全新的LGA 1366接口,45nm製程,集成三通道DDR3內存控制器(支持DDR3 800/1066/1333/1600內存規格),使用新總線QPI與處理器進行連接,支持SMT(Simultaneous Muti-hreading,單顆處理器就可以支持8線程並行技術)多線程技術,支持SSE4.2指令集(增加了7條新的SSE4指令),是intel第一款原生四核處理器……
  當然,在其擁有的眾多技術中,最引人注目的應該還是QPI(原先宣傳的CSI總線)總線技術,他是全新的Nahalem架構之所以能在架構、功能和性能上取得大突破的關鍵性技術。

QPI能給我們帶來什麼

  QPI(Quick Path Interconnect)——"快速通道互聯",取代前端總線(FSB)的一種點到點連接技術,20位寬的QPI連接其帶寬可達驚人的每秒25.6GB,遠非FSB可比。QPI最初能夠發放異彩的是支持多個處理器的服務器平台,QPI可以用於多處理器之間的互聯。
  1. QPI是通信更加方便
  QPI是在處理器中集成內存控制器的體系架構,主要用於處理器之間和系統組件之間的互聯通信(諸如I/O)。他拋棄了沿用多年的的FSB,CPU可直接通過內存控制器訪問內存資源,而不是以前繁雜的「前端總線——北橋——內存控制器」模式。並且,與AMD在主流的多核處理器上採用的4HT3(4根傳輸線路,兩根用於數據發送,兩個用於數據接收)連接方式不同,英特爾採用了4+1 QPI互聯方式(4針對處理器,1針對I/O設計),這樣多處理器的每個處理器都能直接與物理內存相連,每個處理器之間也能彼此互聯來充分利用不同的內存,可以讓多處理器的等待時間變短(訪問延遲可以下降50%以上),只用一個內存插槽就能實現與四路AMD皓龍處理器(AMD在服務器領域的處理器,與intel至強同等產品定位)同等帶寬。
  2. QPI、處理器間峰值帶寬可達96GB/s
  在intel高端的安騰處理器系統中,QPI高速互聯方式使得CPU與CPU之間的峰值帶寬可達96GB/s,峰值內存帶寬可達34GB/s。這主要在於QPI採用了與PCI-E類似的點對點設計,包括一對線路,分別負責數據發送和接收,每一條通路可傳送20bit數據。這就意味著即便是最早的QPI標準,其傳輸速度也能達到6.4GB/s——總計帶寬可達到25.6GB/s(為FSB 1600MHz的12.8GB/S的兩倍)。這樣的帶寬已可媲美AMD目前的總線解決方案,能滿足未來CPU與CPU、CPU與芯片的數據傳輸要求。
  3. 多核間互傳資料不用經過芯片組
  QPI總線可實現多核處理器內部的直接互聯,而無須像以前那樣還要再經過FSB進行連接。例如,針對服務器的Nehalem架構的處理器擁有至少4組QPI傳輸,可至少組成包括4顆處理器的4路高端服務器系統(也就是16顆運算內核至少32線程並行運作)。而且在多處理器作業下,每顆處理器可以互相傳送資料,並不需要經過芯片組,從而大幅提升整體系統性能。隨著未來Nehalem架構的處理器集成內存控制器、PCI-E 2.0圖形接口乃至圖形核心的出現,QPI架構的優勢見進一步發揮出來。
  4. QPI互聯架構本身具有升級性
  QPI採用串聯方式作為訊號的傳送,採用了LVDS(低電壓差分信號技術,主要用於高速數字信號互聯,使信號能以幾百Mbps以上的速率傳輸)信號技術,可保證在高頻率下仍能保持穩定性。QPI擁有更低的延遲及更好的架構,將包括集成的存儲器控制器以及系統組件間的通信鏈路。
  5. QPI總線架構具備可靠性和性能
  可靠性、實用性和適用性特點為QPI的高可用性提供了保證。比如鏈接級循環冗餘碼驗證(CRC)。出現時鐘密碼故障時,時鐘能自動改路發送到數據信道。QPI還具備熱插拔。深度改良的微架構、集成內存控制器設計以及QPI直接技術,令Nehalem擁有更出色的執行效率,在單線程同頻率下,Nehalem擁有更為出色的執行效率,在單線程同頻率條件下,Nehalem的運算能力在相同功耗下比現行的Penryn架構的效能可能提高30%。

QPI對AMD和NVIDIA的影響

  做為行業領導性廠商,每次Intel平台的進步都是有人歡喜有人愁。比如,AMD面臨著該如何追趕Intel處理器革新速度的問題,如果未來AMD無法跟上英特爾的步伐,其市場份額肯定將變得越來越小。當然,AMD有其過硬的顯卡技術支撐,這正是目前Intel所欠缺的。
  AMD CPU如真能將其GPU整合,帶來的市場影響力也是巨大的。
  NVIDIA的處境,Intel的目標是CPU整合GPU,而NVIDIA的目標則是GPU整合CPU,雖然NVIDIA自身對其信心滿滿,從目前的競爭形勢來看,一項是靠顯卡技術、芯片組維繫的NVIDIA,面對Intel的打壓,必須在Intel平台推廣SLI,面對Intel和AMD的CPU整合GPU方案,對NVIDIA的低端、中低端顯卡市場又非常大的影響。

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