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2.2. perf kvm
You can use the
perf
command with the kvm
option to collect guest operating system statistics from the host.
In Red Hat Enterprise Linux, the perf package provides the
perf
command. Run rpm -q perf
to see if the perf package is installed. If it is not installed, and you want to install it to collect and analyze guest operating system statistics, run the following command as the root user:
yum install perf
In order to use
perf kvm
in the host, you must have access to the /proc/modules
and /proc/kallsyms
files from the guest. There are two methods to achieve this. Refer to the following procedure, Procedure 2.1, “Copying /proc files from guest to host” to transfer the files into the host and run reports on the files. Alternatively, refer to Procedure 2.2, “Alternative: using sshfs to directly access files” to directly mount the guest and access the files.
Procedure 2.1. Copying /proc files from guest to host
Important
If you directly copy the required files (for instance, via
scp
) you will only copy files of zero length. This procedure describes how to first save the files in the guest to a temporary location (with the cat
command), and then copy them to the host for use by perf kvm
.
Log in to the guest and save files
Log in to the guest and save/proc/modules
and/proc/kallsyms
to a temporary location,/tmp
:# cat /proc/modules > /tmp/modules # cat /proc/kallsyms > /tmp/kallsyms
Copy the temporary files to the host
Once you have logged off from the guest, run the following examplescp
commands to copy the saved files to the host. You should substitute your host name and TCP port if they are different:# scp root@GuestMachine:/tmp/kallsyms guest-kallsyms # scp root@GuestMachine:/tmp/modules guest-modules
You now have two files from the guest (guest-kallsyms
andguest-modules
) on the host, ready for use byperf kvm
.Recording and reporting events with perf kvm
Using the files obtained in the previous steps, recording and reporting of events in the guest, the host, or both is now possible.Run the following example command:# perf kvm --host --guest --guestkallsyms=guest-kallsyms \ --guestmodules=guest-modules record -a -o perf.data
Note
If both --host and --guest are used in the command, output will be stored inperf.data.kvm
. If only --host is used, the file will be namedperf.data.host
. Similarly, if only --guest is used, the file will be namedperf.data.guest
.Pressing Ctrl-C stops recording.Reporting events
The following example command uses the file obtained by the recording process, and redirects the output into a new file,analyze
.perf kvm --host --guest --guestmodules=guest-modules report -i perf.data.kvm \ --force > analyze
View the contents of theanalyze
file to examine the recorded events:# cat analyze # Events: 7K cycles # # Overhead Command Shared Object Symbol # ........ ............ ................. ......................... # 95.06% vi vi [.] 0x48287 0.61% init [kernel.kallsyms] [k] intel_idle 0.36% vi libc-2.12.so [.] _wordcopy_fwd_aligned 0.32% vi libc-2.12.so [.] __strlen_sse42 0.14% swapper [kernel.kallsyms] [k] intel_idle 0.13% init [kernel.kallsyms] [k] uhci_irq 0.11% perf [kernel.kallsyms] [k] generic_exec_single 0.11% init [kernel.kallsyms] [k] tg_shares_up 0.10% qemu-kvm [kernel.kallsyms] [k] tg_shares_up [output truncated...]
Procedure 2.2. Alternative: using sshfs to directly access files
Important
This is provided as an example only. You will need to substitute values according to your environment.# Get the PID of the qemu process for the guest: PID=`ps -eo pid,cmd | grep "qemu.*-name GuestMachine" \ | grep -v grep | awk '{print $1}'` # Create mount point and mount guest mkdir -p /tmp/guestmount/$PID sshfs -o allow_other,direct_io GuestMachine:/ /tmp/guestmount/$PID # Begin recording perf kvm --host --guest --guestmount=/tmp/guestmount \ record -a -o perf.data # Ctrl-C interrupts recording. Run report: perf kvm --host --guest --guestmount=/tmp/guestmount report \ -i perf.data # Unmount sshfs to the guest once finished: fusermount -u /tmp/guestmount