当我在 shell 中运行 time 命令时,time ./myapp
我得到如下输出:
real 0m0.668s
user 0m0.112s
sys 0m0.028s
但是,当我运行命令时,\time -f %e ./myapp
我失去了精度,并且得到:
2.01s
如果我使用该%E
命令,我也会以同样的方式失去精度。如何更改它以再次获得更高的精度,但仍然只输出秒数?
我的研究基于此Linux/Unix 命令:时间等等这个问题
答案1
我假设您了解这两个命令都调用不同版本的时间,对吗?
bash 的内置版本
% time
GNU 时间又名。 /usr/bin/时间
% \time
可以在此处阅读内置time
命令:bash
% help time
time: time [-p] PIPELINE
Execute PIPELINE and print a summary of the real time, user CPU time,
and system CPU time spent executing PIPELINE when it terminates.
The return status is the return status of PIPELINE. The `-p' option
prints the timing summary in a slightly different format. This uses
the value of the TIMEFORMAT variable as the output format.
GNU time
, /usr/bin/time
, 通常比内置的更有用。
至于您的精度问题,本节对此进行了介绍github要点, 具体来说:
为什么 bash 时间比 GNU 时间更精确?
内置的 bash 命令 time 给出了毫秒级的执行精度,而 GNU time(通常是 /usr/bin/time)给出了厘秒级的精度。 times(2) 系统调用以时钟为单位给出时间,100 个时钟 = 1 秒(通常),因此精度类似于 GNU 时间。 bash time 使用什么以便更精确?
Bash 时间内部使用 getrusage(),GNU 时间内部使用 times()。由于微秒分辨率, getrusage() 更加精确。
您可以通过以下示例查看厘秒(请参阅输出的第 5 行):
% /usr/bin/time -v sleep .22222
Command being timed: "sleep .22222"
User time (seconds): 0.00
System time (seconds): 0.00
Percent of CPU this job got: 0%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.22
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 1968
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 0
Minor (reclaiming a frame) page faults: 153
Voluntary context switches: 2
Involuntary context switches: 1
Swaps: 0
File system inputs: 0
File system outputs: 0
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0
使用 bash 的命令可以获得更高的分辨率,time
如下所示,您可以控制分辨率:
# 3 places
% TIMEFORMAT='%3R'; time ( sleep .22222 )
0.224
来自Bash 变量手册:
TIMEFORMAT
The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed. The ‘%’ character introduces an escape sequence that is expanded to a time value or other information. The escape sequences and their meanings are as follows; the braces denote optional portions.
%%
A literal ‘%’.
%[p][l]R
The elapsed time in seconds.
%[p][l]U
The number of CPU seconds spent in user mode.
%[p][l]S
The number of CPU seconds spent in system mode.
%P
The CPU percentage, computed as (%U + %S) / %R.
The optional p is a digit specifying the precision, the number of fractional digits after a decimal point. A value of 0 causes no decimal point or fraction to be output. At most three places after the decimal point may be specified; values of p greater than 3 are changed to 3. If p is not specified, the value 3 is used.
The optional l specifies a longer format, including minutes, of the form MMmSS.FFs. The value of p determines whether or not the fraction is included.
If this variable is not set, Bash acts as if it had the value
$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
If the value is null, no timing information is displayed. A trailing newline is added when the format string is displayed.