从源码来看，这个参数 vm.dirty_writeback_centisecs 的默认值是5s没错，同时也能看出来参数的单位是以 百分之一秒 作为计量单位的。
https://github.com/torvalds/linux/blob/f079ab01b5609fb0c9acc52c88168bf1eed82373/mm/page-writeback.c#L101
https://github.com/torvalds/linux/blob/1c52283265a462a100ae63ddf58b4e5884acde86/kernel/sysctl.c#L2439-L2443
unsigned int dirty_writeback_interval = 5 * 100; /* centiseconds */
这个参数在源码里对应的变量为 dirty_writeback_interval

https://github.com/torvalds/linux/blob/bbdff6d583be718935b613ab2a966cddaadf661f/mm/backing-dev.c#L251-L255

 * This function is used when the first inode for this wb is marked dirty. It
 * wakes-up the corresponding bdi thread which should then take care of the
 * periodic background write-out of dirty inodes. Since the write-out would
 * starts only 'dirty_writeback_interval' centisecs from now anyway, we just
 * set up a timer which wakes the bdi thread up later.
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其他一些信息或可参考这篇文章：http://t.zoukankan.com/gmpy-p-12657801.html