VIA C7-M 找尋最大TDP (Thermal Design Power)

藉由開機顯示的"VIA Esther processor 1000 MHz",可以判斷是Model A,但Model A 又有 770 779兩個型號,頻率都一樣,只差在770最大TDP是5W,779是3.5W。

使用cpuid dump出來的資訊跟datasheet比較還是查不出是哪一款~
PS: cat /proc/cpuinfo 就是把cpuid的16位元 parse成比較易懂的訊息,不太需要自己去比對cpuid
最後去看 e_powersaver的原始碼,發現他會列出最大的voltage,根據datasheet,5W那顆highest voltage 為0.844 W,3.5W的則為0.796 W
insmod e_powersaver.ko之後,dmesg | grep -i "voltage" 可以看到最大電壓為 796 mV
嗚哇,好奇怪啊!神奇的CPU,如果確定最大是3.5W的話,之前的量測結果為

Frequency(MHz) 400 500 600 700 800 900 1000
CPU Power(W) 0.68 1.1 1.52 1.94 2.42 2.96 3.5

因為我只能量到整個系統的耗電量,所以我假設除了CPU以外的元件耗電量不會隨著頻率而變,然後以1G Hz最多耗3.5 W來估計。

沒想到400 MHz耗電量那麼小,而且根據 W=V2CF的公式算出來,此直線會穿過y軸的負值,也就是0 Hz時,會得到負的耗電量,當然0 Hz到400 Hz不確定能不能這樣推,但如果是當頻率為1G時需消耗5W的話直線會很接近原點。

第二點讓我覺得有點疑惑的地方在於datasheet裡面表示
HALT@50度C (C1)、 StopGrant@50度C (C2)、Sleep@50度C 都需要耗1W的電
然後deepsleep@35度C (C3) 也需要耗 0.75 W,我用400 MHz跑大量的乘法沒道理比他們還省電啊?
所以我潛意識還是認為5W比較合理,但目前沒想法要怎麼證明了~

PS:量測的推論假設除了CPU之外的耗電量不會隨著降頻而改變,除非推論錯了,或者是第二點datasheet寫錯了XD


上面的推論如果是錯的,代表我把TDP3.5 W直接等於CPU最多耗3.5 W,所以要確定一下TDP的定義,看下面的解釋,應該是說CPU實際耗的會大於3.5 W。

TDP的定義:

處理器的功耗與TDP兩者的關係可以用下面公式概括:處理器的功耗=實際消耗功耗+TDP。實際消耗功耗是處理器各個功能單元正常工作消耗的電能,TDP是電流熱效應以及其他形式產生的熱能,他們均以熱的形式釋放。從這個等式我們可以得出這樣的結論:TDP並不等於是處理器的功耗,TDP要小於處理器的功耗。

OK,解決,只是這樣將來就不能只看Datasheet就知道CPU大概的耗電量了,因為TDP每家的定義又不一樣,像AMD,可能是TDP>=CPU所耗的功,因為怕CPU燒掉~

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