monotonic time 该时间自系统开机后就一直单调地增加，它不像xtime可以因用户的调整时间而产生跳变，不过该时间不计算系统休眠的时间，也就是说，系统休眠时，monotoic时间不会递增。
raw monotonic time 该时间与monotonic时间类似，也是单调递增的时间，唯一的不同是：raw monotonic time“更纯净”，他不会受到NTP时间调整的影响，它代表着系统独立时钟硬件对时间的统计。
boot time 与monotonic时间相同，不过会累加上系统休眠的时间，它代表着系统上电后的总时间。
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时间种 类 精度（统计单位） 访问速度 累计休眠时间 受NTP调整的影响
RTC 低 慢 Yes Yes
xtime 高 快 Yes Yes
monotonic 高 快 No Yes
raw monotonic 高 快 No No
boot time 高 快 Yes Yes
2. struct timekeeper
内核用timekeeper结构来组织与时间相关的数据，它的定义如下：
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struct timekeeper {
struct clocksource *clock; /* Current clocksource used for timekeeping. */
u32 mult; /* NTP adjusted clock multiplier */
int shift; /* The shift value of the current clocksource. */
cycle_t cycle_interval; /* Number of clock cycles in one NTP interval. */
u64 xtime_interval; /* Number of clock shifted nano seconds in one NTP interval. */
s64 xtime_remainder; /* shifted nano seconds left over when rounding cycle_interval */
u32 raw_interval; /* Raw nano seconds accumulated per NTP interval. */
u64 xtime_nsec; /* Clock shifted nano seconds remainder not stored in xtime.tv_nsec. */
/* Difference between accumulated time and NTP time in ntp
* shifted nano seconds. */
s64 ntp_error;
/* Shift conversion between clock shifted nano seconds and
* ntp shifted nano seconds. */
int ntp_error_shift;
struct timespec xtime; /* The current time */
struct timespec wall_to_monotonic;
struct timespec total_sleep_time; /* time spent in suspend */
struct timespec raw_time; /* The raw monotonic time for the CLOCK_MONOTONIC_RAW posix clock. */
ktime_t offs_real; /* Offset clock monotonic -> clock realtime */
ktime_t offs_boot; /* Offset clock monotonic -> clock boottime */
seqlock_t lock; /* Seqlock for all timekeeper values */
};
unsigned int jiffies_to_msecs(const unsigned long j);
unsigned int jiffies_to_usecs(const unsigned long j);
unsigned long msecs_to_jiffies(const unsigned int m);
unsigned long usecs_to_jiffies(const unsigned int u);
PSSH provides parallel versions of OpenSSH and related tools. Included are pssh, pscp, prsync, pnuke, and pslurp. The project includes psshlib which can be used within custom applications. The source code is written in Python and can be cloned from:
PSSH is supported on Python 2.4 and greater (including Python 3.1 and greater). It was originally written and maintained by Brent N. Chun. Due to his busy schedule, Brent handed over maintenance to Andrew McNabb in October 2009.
