2.3. 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 command:
yum install systemtap tuned-utils kernel-debuginfo
Run the tools with the command:
diskdevstat
or the command:
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
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 resembles that of PowerTOP. Here is sample output from a longer diskdevstat run on a Fedora 10 system running KDE 4.2:
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
15494 0 sda1 0 0.000 0.000 0.000 758 0.000 0.012 0.000 0logwatch
15520 0 sda1 0 0.000 0.000 0.000 140 0.000 0.009 0.000 perl
15549 0 sda1 0 0.000 0.000 0.000 140 0.000 0.009 0.000 perl
15585 0 sda1 0 0.000 0.000 0.000 108 0.001 0.002 0.000 perl
2573 0 sda1 63 0.033 3600.015 515.226 0 0.000 0.000 0.000 auditd
15429 0 sda1 0 0.000 0.000 0.000 62 0.009 0.009 0.000 crond
15379 0 sda1 0 0.000 0.000 0.000 62 0.008 0.008 0.000 crond
15473 0 sda1 0 0.000 0.000 0.000 62 0.008 0.008 0.000 crond
15415 0 sda1 0 0.000 0.000 0.000 62 0.008 0.008 0.000 crond
15433 0 sda1 0 0.000 0.000 0.000 62 0.008 0.008 0.000 crond
15425 0 sda1 0 0.000 0.000 0.000 62 0.007 0.007 0.000 crond
15375 0 sda1 0 0.000 0.000 0.000 62 0.008 0.008 0.000 crond
15477 0 sda1 0 0.000 0.000 0.000 62 0.007 0.007 0.000 crond
15469 0 sda1 0 0.000 0.000 0.000 62 0.007 0.007 0.000 crond
15419 0 sda1 0 0.000 0.000 0.000 62 0.008 0.008 0.000 crond
15481 0 sda1 0 0.000 0.000 0.000 61 0.000 0.001 0.000 crond
15355 0 sda1 0 0.000 0.000 0.000 37 0.000 0.014 0.001 laptop_mode
2153 0 sda1 26 0.003 3600.029 1290.730 0 0.000 0.000 0.000 rsyslogd
15575 0 sda1 0 0.000 0.000 0.000 16 0.000 0.000 0.000 cat
15581 0 sda1 0 0.000 0.000 0.000 12 0.001 0.002 0.000 perl
15582 0 sda1 0 0.000 0.000 0.000 12 0.001 0.002 0.000 perl
15579 0 sda1 0 0.000 0.000 0.000 12 0.000 0.001 0.000 perl
15580 0 sda1 0 0.000 0.000 0.000 12 0.001 0.001 0.000 perl
15354 0 sda1 0 0.000 0.000 0.000 12 0.000 0.170 0.014 sh
15584 0 sda1 0 0.000 0.000 0.000 12 0.001 0.002 0.000 perl
15548 0 sda1 0 0.000 0.000 0.000 12 0.001 0.014 0.001 perl
15577 0 sda1 0 0.000 0.000 0.000 12 0.001 0.003 0.000 perl
15519 0 sda1 0 0.000 0.000 0.000 12 0.001 0.005 0.000 perl
15578 0 sda1 0 0.000 0.000 0.000 12 0.001 0.001 0.000 perl
15583 0 sda1 0 0.000 0.000 0.000 12 0.001 0.001 0.000 perl
15547 0 sda1 0 0.000 0.000 0.000 11 0.000 0.002 0.000 perl
15576 0 sda1 0 0.000 0.000 0.000 11 0.001 0.001 0.000 perl
15518 0 sda1 0 0.000 0.000 0.000 11 0.000 0.001 0.000 perl
15354 0 sda1 0 0.000 0.000 0.000 10 0.053 0.053 0.005 lm_lid.sh
The columns are:
- 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
In this example, three very obvious 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
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 of course 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. In the present example, you could run:
strace -p 2789
In this example, the output of the strace contained 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.
