What does the numbers at the beginning of prink() mean?

If you use prink() to print kernel message and read it in the console, it looks like this:

<6>[ 2809.666228] amp_enable: amp enable bypass(2)  
<6>[ 2809.666747] amp_enable: AMP_EN is set to 0  
<3>[ 2810.084296] init: untracked pid 4196 exited  
<3>[ 2810.873706] init: untracked pid 4817 exited  
<6>[ 2810.933923] msm_ta_detect_work: USB exit ta detection - frindex  
<6>[ 2817.483839] amp_enable: AMP_EN is set to 1  
<6>[ 2823.084022] adjust_soc: ibat_ua = -114500, vbat_uv = 4296066, soc = 95, batt_temp=302  
<6>[ 2823.669799] SLIM_CL: skip reconfig sequence  
<6>[ 2823.685578] amp_enable: amp enable bypass(2)  
<6>[ 2823.686372] amp_enable: AMP_EN is set to 0

What does the number at the beginning of each line mean? Is it some kind of time stamp? How do I interpret it?

Solution

As NG mentioned, the <3> and <6> are log levels where <3> is KERN_ERR and <6> is KERN_INFO.

Here's a list picked up from http://tuxthink.blogspot.com/2012/07/printk-and-console-log-level.html.

0 KERN_EMERG
1 KERN_ALERT
2 KERN_CRIT
3 KERN_ERR
4 KERN_WARNING
5 KERN_NOTICE
6 KERN_INFO
7 KERN_DEBUG

The next number appears to be time in seconds since system boot. Did your system boot ~50 minutes ago when you saw these messages? The timestamps can help us track how long a task took. For example, amp_enable: amp enable bypass(2) took 0.519ms to complete. From first entry to the fourth entry, it took 1.207478s.

<6>[ 2809.666228] amp_enable: amp enable bypass(2)  
<6>[ 2809.666747] amp_enable: AMP_EN is set to 0  
<3>[ 2810.084296] init: untracked pid 4196 exited  
<3>[ 2810.873706] init: untracked pid 4817 exited

I learned this by visiting http://elinux.org/Printk_Times_Sample1.