为什么这些时间戳在 Perl Time::HiRes 中乱序？

Question

我发现 Perl 的 Time::HiRes 模块报告的时间戳有一些奇怪的行为。

我有一个获取三个时间戳的脚本：

使用 Time::HiRes::time
新建一个文件，修改时间为Time::HiRes::stat
使用 Time::HiRes::time

我希望时间戳是有序的1 < 2 < 3，但情况并非总是如此；通常（但不总是），stat 在 2. 中报告的时间是 before 来自 1..

的时间戳

我使用的是 Ext4 文件系统。这是一个实验：

use Time::HiRes qw/ time stat /;

while( 1 ){
    # t0
    my $t0 = time;

    # Create a file
    my $f = '/tmp/dummy.test';
    open(my $fh, '>', $f) || die;
    print $fh "hi\n";
    close($fh) || die;

    # FS: file modification time, according to the filestystem
    my $fs = (stat($f))[9];

    # t1
    my $t1 = time;

    ## Report how the timestamps relate to each other
    # A. All good
    if( $t0 < $fs && $fs < $t1 ){
       print "$t1,0\n";
    }
    # B. FS before t0
    elsif( $t0 > $fs && $fs < $t1 ){
       print "$t1,1\n";
    }
    # C. FS after t1
    elsif( $t0 < $fs && $fs > $t1 ){
       print "$t1,2\n";
    }
    # D. this should never happen (t0 and t1 probably can't flip)
    elsif( $t0 > $fs && $fs > $t1 ){
       print "$t1,3\n";
    }
}

下面是让上面的循环运行几秒钟的结果。底部的蓝点是 "correct" 行为的事件。通常情况下，我得到条件 B，其中 stat 的修改时间是在第一个时间戳之前。

什么可以解释这种行为？

更新： 这是前 2000 次迭代的时间戳滞后图：

Answer 1

这可能是因为文档中提到的两个时间戳的精度不同 here:

As stat or lstat but with the access/modify/change file timestamps in subsecond resolution, if the operating system and the filesystem both support such timestamps. To override the standard stat():
use Time::HiRes qw(stat);
Test for the value of &Time::HiRes::d_hires_stat to find out whether the operating system supports subsecond file timestamps: a value larger than zero means yes. There are unfortunately no easy ways to find out whether the filesystem supports such timestamps. UNIX filesystems often do; NTFS does; FAT doesn't (FAT timestamp granularity is two seconds).

A zero return value of &Time::HiRes::d_hires_stat means that Time::HiRes::stat is a no-op passthrough for CORE::stat() (and likewise for lstat), and therefore the timestamps will stay integers. The same thing will happen if the filesystem does not do subsecond timestamps, even if the &Time::HiRes::d_hires_stat is non-zero.

In any case do not expect nanosecond resolution, or even a microsecond resolution. Also note that the modify/access timestamps might have different resolutions, and that they need not be synchronized, e.g. if the operations are
write
stat # t1
read
stat # t2
the access time stamp from t2 need not be greater-than the modify time stamp from t1: it may be equal or less.

为什么这些时间戳在 Perl Time::HiRes 中乱序？

Why are these timestamps out of order with Perl Time::HiRes?

perl

unix

timestamp

ext4