[理工] [os] 94清大

[compulsory (まけない!)]

A computer whose processes have 1024 pages in their address spaces keeps it's
page tables in memory. The overhead reading aword from the page table is 500ns
. In order to reduce the overhead, the computer has a TLB, which holds 32
(virtual page,physical page frame)pairs, and can do a look up in 100ns.
What hit rate is needed to reduce the mean overhead to 200ns?

個人的想法是:
設P為TLB的hit rate

如果TLB hit 存取時間100+500=600ns

如果TLB miss
存取TLB的時間加上存取一次page table加上一次memory的時間
100+500+500=1100ns

P*600+(1-P)*1100=200

可是參考書的答案是

P*100+(1-P)*600=200

我哪裡想錯了??

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◆ From: 122.116.13.191
→ compulsory:照我的方法一定錯  解出來P>1
推 christianSK:我不懂為什麼你TLB的hit time要加上page table的時間
→ christianSK:TLB如果hit 就不需要對page table作存取了吧
→ compulsory:不是TLB hit 還要到MEMORY去抓資料嗎?
→ compulsory:我以為題目說的page table存取時間當作memory存取時間
推 christianSK:應該是因為overhead吧 存取資料時的額外付出
→ christianSK:如果是這樣的話 我想就不需要考慮讀取資料的時間
→ compulsory:我懂了  謝謝<(_ _)>