php-fpm太多关于跟踪的注意事项

Question

I am doing a web performance test for nginx+php-fpm on linux，and env is:

linux: linux version 2.6.32-358.el6.x86_64 ([e-mail]) (gcc version 4.4.7 20120313 (Red Hat 4.4.7-3) (GCC) )
nginx : 1.4.6
php: 5.4.26

and I keep on sending 100 concurrent request for a long time， and then I see many php-fpm logs like below：

[12-Mar-2015 12:16:37] NOTICE: finished trace of 16926
[12-Mar-2015 12:16:37] NOTICE: child 16927 stopped for tracing
[12-Mar-2015 12:16:37] NOTICE: about to trace 16927
[12-Mar-2015 12:16:37] NOTICE: finished trace of 16927
[12-Mar-2015 12:16:37] NOTICE: child 16928 stopped for tracing
[12-Mar-2015 12:16:37] NOTICE: about to trace 16928
[12-Mar-2015 12:16:37] NOTICE: finished trace of 16928
[12-Mar-2015 12:16:37] NOTICE: child 16929 stopped for tracing
[12-Mar-2015 12:16:37] NOTICE: about to trace 16929
[12-Mar-2015 12:16:37] NOTICE: finished trace of 16929
[12-Mar-2015 12:16:37] NOTICE: child 16930 stopped for tracing
[12-Mar-2015 12:16:37] NOTICE: about to trace 16930
[12-Mar-2015 12:16:37] NOTICE: finished trace of 16930
[12-Mar-2015 12:16:37] NOTICE: child 16931 stopped for tracing
[12-Mar-2015 12:16:37] NOTICE: about to trace 16931
[12-Mar-2015 12:16:37] NOTICE: finished trace of 16931
[12-Mar-2015 12:16:37] NOTICE: child 16932 stopped for tracing
[12-Mar-2015 12:16:37] NOTICE: about to trace 16932
[12-Mar-2015 12:16:37] NOTICE: finished trace of 16932

So what's this and what does this means?