推 liataian:剛剛翻一下恐龍才發現原來這是恐龍題目 冏 09/15 20:12
→ jackbll:推~ 09/15 20:31
推 da0910cc: 09/15 22:23
※ 引述《liataian (柴油)》之銘言：
: 板友好 想請教一下
: 中央有一題問說:
: 為何spinlocks不適用於single-processor系統
: 而是常使用於multiprocessor系統
: 我去google了一下有goo到答案 題目一模一樣!? 如下
: Q:Why spinlocks are not appropriate for single-processor systems
: yet are often used in multiprocessor systems?
: A:"Spinlocks should not be used on single-processor systems. Consider a high
: priority thread which attempts to claim a spinlock already held by a lower
: priority thread: it will just loop forever and the lower priority thread will
: never get another chance to run and release the spinlock. Even if the two
: threads were running at the same priority, the one attempting to claim the
: spinlock would spin until it was timesliced and a lot of cpu time would be
: wasted. If an interrupt handler tried to claim a spinlock owned by a thread,
: the interrupt handler would loop forever. Therefore spinlocks are only
: appropriate for SMP systems where the current owner of a spinlock can
: continue running on a different processor. "
: 老實說有點有看沒有很懂
: 有板友能用淺顯一點的話幫我解釋一下嗎 謝謝> <
: 還是說有更簡潔的說法 不用那麼複雜呢?
假如只有一個processor，
process P1正霸占著CPU spin，
在P1的time slice之中解除spin的條件就永遠不能滿足(因為就只有P1自己在那邊spin)，
P1就在那邊一直spin spin spin spin...
直到P1的time slice用完，
這樣分配給P1的整個time slice就浪費掉了。
但是若有兩個以上的processors，
process P1在processor A上spin，
在其他processors上同時執行的其他processes可能可以讓P1從spinlock中解脫，
這時P1可以繼續做些有用的事情，而不是浪費整個time slice在spin。
有錯誤的話還請多指正，謝謝 :)
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