When running a latency-sensitive application one might notice that on a regular basis (for example every 5 minutes) there is a delay. The SystemTap
periodic.stp
script can provide some possible causes of that regular delay. The SystemTap periodic.stp
script generates a list of the number of times that various scheduled functions run and the time between each scheduled execution. In the case of delay every five minutes one would run the periodic script for tens of minutes and then look through the output list for a function that has a period of approximately 300,000,00 microseconds (5 minutes60seconds1,000,000 microseconds/second).
Follow the instructions in the SystemTap Beginners Guide to install SystemTap. The SystemTap
--all-modules
option should be included on the command line when using periodic.stp
script so the script can map the kernel module instruction addresses back to function names and make it easier to interpret what is periodically running. The periodic script will collect data until Ctrl+c is pressed. Below is an example run of periodic.stp
:
Raw
$ /usr/share/doc/systemtap-*/examples/profiling/periodic.stp --all-modules
#monitoring timer periods (press control-c for output)
^C#type function period(us) count
kernel 0xffffffffa0c59200 [stap_eb782113d30627ea6aa3ef949 1000 13552
kernel 0xffffffffa0c59200 [stap_eb782113d30627ea6aa3ef949 1000 13552
kernel 0xffffffffa0c59200 [stap_eb782113d30627ea6aa3ef949 1000 13551
kernel 0xffffffffa0c59200 [stap_eb782113d30627ea6aa3ef949 1000 13550
work_q do_dbs_timer 9999 1354
work_q do_dbs_timer 9999 1354
kernel 0xffffffffa0c590e0 [stap_eb782113d30627ea6aa3ef949 9999 1354
work_q do_dbs_timer 10000 1354
work_q do_dbs_timer 10022 1351
kernel 0xffffffffa0c59020 [stap_eb782113d30627ea6aa3ef949 19999 676
kernel monitor_timeout+0x0/0x10 [intel_ips] 200003 67
kernel clocksource_watchdog+0x0/0x230 [kernel] 500033 26
kernel tcp_delack_timer+0x0/0x270 [kernel] 981240 13
work_q i915_gem_retire_work_handler 1007250 12
work_q vmstat_update 1000000 12
kernel ipmi_timeout+0x0/0x470 [kernel] 1000002 12
work_q vmstat_update 1000000 12
kernel br_fdb_cleanup+0x0/0x120 [bridge] 999999 12
work_q vmstat_update 999999 12
work_q vmstat_update 1000000 12
process kslowd000(157) 4983 8
work_q cache_reap 2000001 6
kernel e1000_watchdog+0x0/0x20 [e1000e] 2000001 6
work_q cache_reap 1999999 6
work_q cache_reap 1999998 6
kernel br_hello_timer_expired+0x0/0x90 [bridge] 1999998 6
work_q cache_reap 2000000 6
kernel tcp_write_timer+0x0/0x200 [kernel] 2933662 3
kernel cfq_idle_slice_timer+0x0/0x100 [kernel] 2037998 3
work_q flush_to_ldisc 3017627 3
kernel dev_watchdog+0x0/0x280 [kernel] 5000425 2
process bdi-default(29) 5000211 2
process ips-adjust(892) 5000047 2
kernel sync_supers_timer_fn+0x0/0x20 [kernel] 6000000 2
kernel inet_twdr_hangman+0x0/0xd0 [kernel] 7499776 1
process kslowd000(157) 1126 1
process flush-253:0(1336) 4999981 1
work_q do_cache_clean 4875 1
process flush-253:4(11335) 5000013 1
The first column is the type of queue that the task periodically run and can be a
process
, a
kernel
, or a
work_q
. A
process
entry is a user-space process with the second column listing the process name and the associated PID. A
kernel
entry is a kernel function scheduled to run with the second column listing the function and module name. A
work_q
entry is also a function in kernel-space, but it runs in a process context and is able to sleep. The
period(us)
column lists the measured period in microseconds between starts of the task. Variation in the time measurements may cause a timer interval of 10000 (1 millisecond) to be off, for example 9999 microseconds. The data is collated by the kernel timer data structure. A timer data structure may be initialized for each processor. Thus, for a periodic task running on all processrs of a four processor machine there would be four entries in the
periodic.stp
output (as seen for
do_dbs_timer
above). The last column is the count which indicates the number of times that the function or process ran.