guestfs-performance(1) Virtualization Support guestfs-performance(1)NAMEguestfs-performance - engineering libguestfs for greatest performance
DESCRIPTION
This page documents how to get the greatest performance out of
libguestfs, especially when you expect to use libguestfs to manipulate
thousands of virtual machines or disk images.
Three main areas are covered. Libguestfs runs an appliance (a small
Linux distribution) inside qemu/KVM. The first two areas are:
minimizing the time taken to start this appliance, and the number of
times the appliance has to be started. The third area is shortening
the time taken for inspection of VMs.
BASELINE MEASUREMENTS
Before making changes to how you use libguestfs, take baseline
measurements.
BASELINE: STARTING THE APPLIANCE
On an unloaded machine, time how long it takes to start up the
appliance:
time guestfish -a /dev/null run
Run this command several times in a row and discard the first few runs,
so that you are measuring a typical "hot cache" case.
Explanation
This command starts up the libguestfs appliance on a null disk, and
then immediately shuts it down. The first time you run the command, it
will create an appliance and cache it (usually under
"/var/tmp/.guestfs-*"). Subsequent runs should reuse the cached
appliance.
Expected results
You should expect to be getting times under 6 seconds. If the times
you see on an unloaded machine are above this, then see the section
"TROUBLESHOOTING POOR PERFORMANCE" below.
BASELINE: PERFORMING INSPECTION OF A GUEST
For this test you will need an unloaded machine and at least one real
guest or disk image. If you are planning to use libguestfs against
only X guests (eg. X = Windows), then using an X guest here would be
most appropriate. If you are planning to run libguestfs against a mix
of guests, then use a mix of guests for testing here.
Time how long it takes to perform inspection and mount the disks of the
guest. Use the first command if you will be using disk images, and the
second command if you will be using libvirt.
time guestfish --ro -a disk.img -i exit
time guestfish --ro -d GuestName -i exit
Run the command several times in a row and discard the first few runs,
so that you are measuring a typical "hot cache" case.
Explanation
This command starts up the libguestfs appliance on the named disk image
or libvirt guest, performs libguestfs inspection on it (see
"INSPECTION" in guestfs(3)), mounts the guest's disks, then discards
all these results and shuts down.
The first time you run the command, it will create an appliance and
cache it (usually under "/var/tmp/.guestfs-*"). Subsequent runs should
reuse the cached appliance.
Expected results
You should expect times which are X 5 seconds greater than measured in
the first baseline test above. (For example, if the first baseline
test ran in 5 seconds, then this test should run in X 10 seconds).
UNDERSTANDING THE APPLIANCE AND WHEN IT IS BUILT/CACHED
The first time you use libguestfs, it will build and cache an
appliance. This is usually in "/var/tmp/.guestfs-*", unless you have
set $TMPDIR or $LIBGUESTFS_CACHEDIR in which case it will be under that
temporary directory.
For more information about how the appliance is constructed, see
"SUPERMIN APPLIANCES" in supermin(8).
Every time libguestfs runs it will check that no host files used by the
appliance have changed. If any have, then the appliance is rebuilt.
This usually happens when a package is installed or updated on the host
(eg. using programs like "yum" or "apt-get"). The reason for
reconstructing the appliance is security: the new program that has been
installed might contain a security fix, and so we want to include the
fixed program in the appliance automatically.
These are the performance implications:
· The process of building (or rebuilding) the cached appliance is
slow, and you can avoid this happening by using a fixed appliance
(see below).
· If not using a fixed appliance, be aware that updating software on
the host will cause a one time rebuild of the appliance.
· "/var/tmp" (or $TMPDIR, $LIBGUESTFS_CACHEDIR) should be on a fast
disk, and have plenty of space for the appliance.
USING A FIXED APPLIANCE
To fully control when the appliance is built, you can build a fixed
appliance. This appliance should be stored on a fast local disk.
To build the appliance, run the command:
libguestfs-make-fixed-appliance <directory>
replacing "<directory>" with the name of a directory where the
appliance will be stored (normally you would name a subdirectory, for
example: "/usr/local/lib/guestfs/appliance" or "/dev/shm/appliance").
Then set $LIBGUESTFS_PATH (and ensure this environment variable is set
in your libguestfs program), or modify your program so it calls
"guestfs_set_path". For example:
export LIBGUESTFS_PATH=/usr/local/lib/guestfs/appliance
Now you can run libguestfs programs, virt tools, guestfish etc. as
normal. The programs will use your fixed appliance, and will not ever
build, rebuild, or cache their own appliance.
(For detailed information on this subject, see:
libguestfs-make-fixed-appliance(1)).
PERFORMANCE OF THE FIXED APPLIANCE
In our testing we did not find that using a fixed appliance gave any
measurable performance benefit, even when the appliance was located in
memory (ie. on "/dev/shm"). However there are three points to
consider:
1. Using a fixed appliance stops libguestfs from ever rebuilding the
appliance, meaning that libguestfs will have more predictable
start-up times.
2. By default libguestfs (or rather, supermin-helper(8)) searches over
the root filesystem to find out if any host files have changed and
if it needs to rebuild the appliance. If these files are not
cached and the root filesystem is on an HDD, then this generates
lots of seeks. Using a fixed appliance avoids this.
3. The appliance is loaded on demand. A simple test such as:
time guestfish -a /dev/null run
does not load very much of the appliance. A real libguestfs
program using complicated API calls would demand-load a lot more of
the appliance. Being able to store the appliance in a specified
location makes the performance more predictable.
REDUCING THE NUMBER OF TIMES THE APPLIANCE IS LAUNCHED
By far the most effective, though not always the simplest way to get
good performance is to ensure that the appliance is launched the
minimum number of times. This will probably involve changing your
libguestfs application.
Try to call "guestfs_launch" at most once per target virtual machine or
disk image.
Instead of using a separate instance of guestfish(1) to make a series
of changes to the same guest, use a single instance of guestfish and/or
use the guestfish --listen option.
Consider writing your program as a daemon which holds a guest open
while making a series of changes. Or marshal all the operations you
want to perform before opening the guest.
You can also try adding disks from multiple guests to a single
appliance. Before trying this, note the following points:
1. Adding multiple guests to one appliance is a security problem
because it may allow one guest to interfere with the disks of
another guest. Only do it if you trust all the guests, or if you
can group guests by trust.
2. There is a hard limit to the number of disks you can add to a
single appliance. Call "guestfs_max_disks" in guestfs(3) to get
this limit. For further information see "LIMITS" in guestfs(3).
3. Using libguestfs this way is complicated. Disks can have
unexpected interactions: for example, if two guests use the same
UUID for a filesystem (because they were cloned), or have volume
groups with the same name (but see "guestfs_lvm_set_filter").
virt-df(1) adds multiple disks by default, so the source code for this
program would be a good place to start.
SHORTENING THE TIME TAKEN FOR INSPECTION OF VMs
The main advice is obvious: Do not perform inspection (which is
expensive) unless you need the results.
If you previously performed inspection on the guest, then it may be
safe to cache and reuse the results from last time.
Some disks don't need to be inspected at all: for example, if you are
creating a disk image, or if the disk image is not a VM, or if the disk
image has a known layout.
Even when basic inspection ("guestfs_inspect_os") is required,
auxiliary inspection operations may be avoided:
· Mounting disks is only necessary to get further filesystem
information.
· Listing applications ("guestfs_inspect_list_applications") is an
expensive operation on Linux, but almost free on Windows.
· Generating a guest icon ("guestfs_inspect_get_icon") is cheap on
Linux but expensive on Windows.
PARALLEL APPLIANCES
Libguestfs appliances are mostly I/O bound and you can launch multiple
appliances in parallel. Provided there is enough free memory, there
should be little difference in launching 1 appliance vs N appliances in
parallel.
On a 2-core (4-thread) laptop with 16 GB of RAM, using the (not
especially realistic) test Perl script below, the following plot shows
excellent scalability when running between 1 and 20 appliances in
parallel:
12 ++---+----+----+----+-----+----+----+----+----+---++
+ + + + + + + + + + *
| |
| * |
11 ++ ++
| |
| |
| * * |
10 ++ ++
| * |
| |
s | |
9 ++ ++
e | |
| * |
c | |
8 ++ * ++
o | * |
| |
n 7 ++ ++
| * |
d | * |
| |
s 6 ++ ++
| * * |
| * |
| |
5 ++ ++
| |
| * |
| * * |
4 ++ ++
| |
| |
+ * * * + + + + + + + +
3 ++-*-+----+----+----+-----+----+----+----+----+---++
0 2 4 6 8 10 12 14 16 18 20
number of parallel appliances
It is possible to run many more than 20 appliances in parallel, but if
you are using the libvirt backend then you should be aware that out of
the box libvirt limits the number of client connections to 20.
The simple Perl script below was used to collect the data for the plot
above, but there is much more information on this subject, including
more advanced test scripts and graphs, available in the following blog
postings:
http://rwmj.wordpress.com/2013/02/25/multiple-libguestfs-appliances-in-parallel-part-1/
http://rwmj.wordpress.com/2013/02/25/multiple-libguestfs-appliances-in-parallel-part-2/
http://rwmj.wordpress.com/2013/02/25/multiple-libguestfs-appliances-in-parallel-part-3/
http://rwmj.wordpress.com/2013/02/25/multiple-libguestfs-appliances-in-parallel-part-4/
#!/usr/bin/perl -w
use strict;
use threads;
use Sys::Guestfs;
use Time::HiRes qw(time);
sub test {
my $g = Sys::Guestfs->new;
$g->add_drive_ro ("/dev/null");
$g->launch ();
# You could add some work for libguestfs to do here.
$g->close ();
}
# Get everything into cache.
test (); test (); test ();
for my $nr_threads (1..20) {
my $start_t = time ();
my @threads;
foreach (1..$nr_threads) {
push @threads, threads->create (\&test)
}
foreach (@threads) {
$_->join ();
if (my $err = $_->error ()) {
die "launch failed with $nr_threads threads: $err"
}
}
my $end_t = time ();
printf ("%d %.2f\n", $nr_threads, $end_t - $start_t);
}
TROUBLESHOOTING POOR PERFORMANCE
ENSURE HARDWARE VIRTUALIZATION IS AVAILABLE
Use "/proc/cpuinfo" and this page:
http://virt-tools.org/learning/check-hardware-virt/
to ensure that hardware virtualization is available. Note that you may
need to enable it in your BIOS.
Hardware virt is not usually available inside VMs, and libguestfs will
run slowly inside another virtual machine whatever you do. Nested
virtualization does not work well in our experience, and is certainly
no substitute for running libguestfs on baremetal.
ENSURE KVM IS AVAILABLE
Ensure that KVM is enabled and available to the user that will run
libguestfs. It should be safe to set 0666 permissions on "/dev/kvm"
and most distributions now do this.
PROCESSORS TO AVOID
Avoid processors that don't have hardware virtualization, and some
processors which are simply very slow (AMD Geode being a great
example).
DETAILED TIMINGS USING ANNOTATE
Use the annotate(1)/annotate-output(1) command to show detailed
timings:
$ annotate-output +'%T.%N' guestfish -a /dev/null run -v
22:17:53.215784625 I: Started guestfish -a /dev/null run -v
22:17:53.240335409 E: libguestfs: [00000ms] supermin-helper --verbose -f checksum '/usr/lib64/guestfs/supermin.d' x86_64
22:17:53.266857866 E: supermin helper [00000ms] whitelist = (not specified), host_cpu = x86_64, kernel = (null), initrd = (null), appliance = (null)
22:17:53.272704072 E: supermin helper [00000ms] inputs[0] = /usr/lib64/guestfs/supermin.d
22:17:53.276528651 E: checking modpath /lib/modules/3.4.0-1.fc17.x86_64.debug is a directory
[etc]
The timestamps are "hours:minutes:seconds.nanoseconds". By comparing
the timestamps you can see exactly how long each operation in the boot
sequence takes.
DETAILED TIMINGS USING SYSTEMTAP
You can use SystemTap (stap(1)) to get detailed timings from libguestfs
programs.
Save the following script as "time.stap":
global last;
function display_time () {
now = gettimeofday_us ();
delta = 0;
if (last > 0)
delta = now - last;
last = now;
printf ("%d (+%d):", now, delta);
}
probe begin {
last = 0;
printf ("ready\n");
}
/* Display all calls to static markers. */
probe process("/usr/lib*/libguestfs.so.0")
.provider("guestfs").mark("*") ? {
display_time();
printf ("\t%s %s\n", $$name, $$parms);
}
/* Display all calls to guestfs_* functions. */
probe process("/usr/lib*/libguestfs.so.0")
.function("guestfs_[a-z]*") ? {
display_time();
printf ("\t%s %s\n", probefunc(), $$parms);
}
Run it as root in one window:
# stap time.stap
ready
It prints "ready" when SystemTap has loaded the program. Run your
libguestfs program, guestfish or a virt tool in another window. For
example:
$ guestfish -a /dev/null run
In the stap window you will see a large amount of output, with the time
taken for each step shown (microseconds in parenthesis). For example:
xxxx (+0): guestfs_create
xxxx (+29): guestfs_set_pgroup g=0x17a9de0 pgroup=0x1
xxxx (+9): guestfs_add_drive_opts_argv g=0x17a9de0 [...]
xxxx (+8): guestfs___safe_strdup g=0x17a9de0 str=0x7f8a153bed5d
xxxx (+19): guestfs___safe_malloc g=0x17a9de0 nbytes=0x38
xxxx (+5): guestfs___safe_strdup g=0x17a9de0 str=0x17a9f60
xxxx (+10): guestfs_launch g=0x17a9de0
xxxx (+4): launch_start
[etc]
You will need to consult, and even modify, the source to libguestfs to
fully understand the output.
DETAILED DEBUGGING USING GDB
You can attach to the appliance BIOS/kernel using gdb. If you know
what you're doing, this can be a useful way to diagnose boot
regressions.
Firstly, you have to change qemu so it runs with the "-S" and "-s"
options. These options cause qemu to pause at boot and allow you to
attach a debugger. Read qemu(1) for further information. Libguestfs
invokes qemu several times (to scan the help output and so on) and you
only want the final invocation of qemu to use these options, so use a
qemu wrapper script like this:
#!/bin/bash -
# Set this to point to the real qemu binary.
qemu=/usr/bin/qemu-kvm
if [ "$1" != "-global" ]; then
# Scanning help output etc.
exec $qemu "$@"
else
# Really running qemu.
exec $qemu -S -s "$@"
fi
Now run guestfish or another libguestfs tool with the qemu wrapper (see
"QEMU WRAPPERS" in guestfs(3) to understand what this is doing):
LIBGUESTFS_QEMU=/path/to/qemu-wrapper guestfish -a /dev/null -v run
This should pause just after qemu launches. In another window, attach
to qemu using gdb:
$ gdb
(gdb) set architecture i8086
The target architecture is assumed to be i8086
(gdb) target remote :1234
Remote debugging using :1234
0x0000fff0 in ?? ()
(gdb) cont
At this point you can use standard gdb techniques, eg. hitting "^C" to
interrupt the boot and "bt" get a stack trace, setting breakpoints,
etc. Note that when you are past the BIOS and into the Linux kernel,
you'll want to change the architecture back to 32 or 64 bit.
SEE ALSOsupermin(8), supermin-helper(8), guestfish(1), guestfs(3),
guestfs-examples(3), libguestfs-make-fixed-appliance(1), stap(1),
qemu(1), gdb(1), http://libguestfs.org/.
AUTHORS
Richard W.M. Jones ("rjones at redhat dot com")
COPYRIGHT
Copyright (C) 2012 Red Hat Inc.
LICENSE
This library is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published
by the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
02110-1301 USA
BUGS
To get a list of bugs against libguestfs, use this link:
https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools
To report a new bug against libguestfs, use this link:
https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools
When reporting a bug, please supply:
· The version of libguestfs.
· Where you got libguestfs (eg. which Linux distro, compiled from
source, etc)
· Describe the bug accurately and give a way to reproduce it.
· Run libguestfs-test-tool(1) and paste the complete, unedited output
into the bug report.
libguestfs-1.20.4 2013-03-12 guestfs-performance(1)
