alternative-live

しかし、最近の電力消費量制御機構は、CPU駆動クロック周波数を変更してしまう。すると TSC の値は何がなんだか判らなくなる… そこで、Linux Kernel 内部には cycles_2_ns() という関数があって、TSCの値をナノ秒に修正してくれる事になっている。

しかし、不幸はここに内包されていた

すごく簡単に言うと、TSCの値を必要な Scale Factor 倍すれば　nano sec になる。しかし、TSCの値は nano sec に近い。当然 Scale Factor は 1.0 に近い、微妙な浮動小数点になってしまう。

そうだ! Scale Factor を分数で表そう!! SC/1024 で表現して、 SCを整数にしておけば、 SCを掛け算した後、 右に10bit シフトすれば済むよっ!!!

…この段階で何か嫌な予感がしなかったんだろうか、こいつらは…

“
— okkyの銀河制圧奇譚: sched_clock() overflow after 208.5 days in Linux Kernel (via otsune)
Reblogged 1 month ago from otsune
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Source: kenichiokuyama.blogspot.com
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