Red Hat Summit2.3. diskdevstat and netdevstat
diskdevstat and netdevstat are SystemTap tools that collect detailed information about the disk activity and network activity of all applications running on a system. These tools were inspired by PowerTOP, which shows the number of CPU wakeups by every application per second (refer to Section 2.2, “PowerTOP”). The statistics that these tools collect allow you to identify applications that waste power with many small I/O operations rather than fewer, larger operations. Other monitoring tools that measure only transfer speeds do not help to identify this type of usage.
Install these tools with SystemTap with the following command as
root:
yum install systemtap tuned-utils kernel-debuginfo
yum install systemtap tuned-utils kernel-debuginfo
Run the tools with the command:
diskdevstat
diskdevstat
or the command:
netdevstat
netdevstat
Both commands can take up to three parameters, as follows:
diskdevstat update_interval total_duration display_histogram netdevstat update_interval total_duration display_histogram
diskdevstat update_interval total_duration display_histogramdiskdevstat update_interval total_duration display_histogramdiskdevstat update_interval total_duration display_histogramdiskdevstat update_interval total_duration display_histogramdiskdevstat update_interval total_duration display_histogramdiskdevstat update_interval total_duration display_histogram
netdevstat update_interval total_duration display_histogramnetdevstat update_interval total_duration display_histogramnetdevstat update_interval total_duration display_histogramnetdevstat update_interval total_duration display_histogramnetdevstat update_interval total_duration display_histogramnetdevstat update_interval total_duration display_histogram- update_interval
- The time in seconds between updates of the display. Default:
5 - total_duration
- The time in seconds for the whole run. Default:
86400(1 day) - display_histogram
- Flag whether to histogram for all the collected data at the end of the run.
The output of the
diskdevstat command resembles that of PowerTOP. See the example.
Example 2.1. An Output of the diskdevstat Command
Here is sample output from a longer diskdevstat run:
Three applications stand out:
PID UID DEV WRITE_CNT WRITE_MIN WRITE_MAX WRITE_AVG READ_CNT READ_MIN READ_MAX READ_AVG COMMAND 2789 2903 sda1 854 0.000 120.000 39.836 0 0.000 0.000 0.000 plasma 2573 0 sda1 63 0.033 3600.015 515.226 0 0.000 0.000 0.000 auditd 2153 0 sda1 26 0.003 3600.029 1290.730 0 0.000 0.000 0.000 rsyslogd
PID UID DEV WRITE_CNT WRITE_MIN WRITE_MAX WRITE_AVG READ_CNT READ_MIN READ_MAX READ_AVG COMMAND
2789 2903 sda1 854 0.000 120.000 39.836 0 0.000 0.000 0.000 plasma
2573 0 sda1 63 0.033 3600.015 515.226 0 0.000 0.000 0.000 auditd
2153 0 sda1 26 0.003 3600.029 1290.730 0 0.000 0.000 0.000 rsyslogd
These three applications have a
WRITE_CNT greater than 0, which means that they performed some form of write during the measurement. Of those, plasma was the worst offender by a large degree: it performed the most write operations, and the average time between writes was the lowest. Plasma would therefore be the best candidate to investigate if you were concerned about power-inefficient applications.
Use the strace and ltrace commands to examine applications more closely by tracing all system calls of the given process ID. Run:
strace -p 2789
strace -p 2789
The output of
strace contains a repeating pattern every 45 seconds that opened the KDE icon cache file of the user for writing followed by an immediate close of the file again. This led to a necessary physical write to the hard disk as the file metadata (specifically, the modification time) had changed. The final fix was to prevent those unnecessary calls when no updates to the icons had occurred.
For reference on what the columns in the
diskdevstat command stand for, see this table:
| PID | the process ID of the application |
| UID | the user ID under which the applications is running |
| DEV | the device on which the I/O took place |
| WRITE_CNT | the total number of write operations |
| WRITE_MIN | the lowest time taken for two consecutive writes (in seconds) |
| WRITE_MAX | the greatest time taken for two consecutive writes (in seconds) |
| WRITE_AVG | the average time taken for two consecutive writes (in seconds) |
| READ_CNT | the total number of read operations |
| READ_MIN | the lowest time taken for two consecutive reads (in seconds) |
| READ_MAX | the greatest time taken for two consecutive reads (in seconds) |
| READ_AVG | the average time taken for two consecutive reads (in seconds) |
| COMMAND | the name of the process |
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