guestfs-performance - engineering libguestfs for greatest performance
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.
Before making changes to how you use libguestfs, take baseline measurements.
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.
Side note for developers: There is a program called
boot-benchmark
in
https://github.com/libguestfs/libguestfs-analysis-tools which does the same
thing, but performs multiple runs and prints the mean and standard deviation.
説明
The guestfish command above 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.
期待される結果
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.
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.
説明
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.
期待される結果
You should expect times which are ≤ 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 ≤ 10 seconds).
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(1).
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.
To fully control when the appliance is built, you can build a fixed appliance.
This appliance should be stored on a fast local disk.
アプライアンスを構築するには、以下のコマンドを実行します:
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.
(この話題の詳細は
libguestfs-make-fixed-appliance(1)
を参照してください)。
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 two 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.
- 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.
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.
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.
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/env perl
use strict;
use threads;
use warnings;
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);
}
Use
/proc/cpuinfo to ensure that hardware virtualization is available.
Note that you may need to enable it in your BIOS.
ハードウェア仮想化は一般的に仮想マシンの中において利用可能ではありません。libguestfs
はどんな他の仮想マシンよりも遅いです。ネスト仮想化は経験上うまく動作しないです。そのため、ベアメタルにおいて
libguestfs
を実行するためにほとんど適切ではありません。
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.
Avoid processors that don’t have hardware virtualization, and some
processors which are simply very slow (AMD Geode being a great example).
In Xen, dom0 is a virtual machine, and so hardware virtualization is not
available.
During the libguestfs 1.33 development cycle, we spent a large amount of time
concentrating on boot performance, and added some patches to libguestfs, qemu
and Linux which in some cases can reduce boot times to well under 1 second.
You may therefore get much better performance by moving to the versions of
libguestfs or qemu mentioned in the heading.
In
https://github.com/libguestfs/libguestfs-analysis-tools is a program called
"boot-analysis". This program is able to produce a very detailed
breakdown of the boot steps (eg. qemu, BIOS, kernel, libguestfs init script),
and can measure how long it takes to perform each step.
Use the
ts(1) command (from moreutils) to show detailed timings:
$ guestfish -a /dev/null run -v |& ts -i '%.s'
0.000022 libguestfs: launch: program=guestfish
0.000134 libguestfs: launch: version=1.29.31fedora=23,release=2.fc23,libvirt
0.000044 libguestfs: launch: backend registered: unix
0.000035 libguestfs: launch: backend registered: uml
0.000035 libguestfs: launch: backend registered: libvirt
0.000032 libguestfs: launch: backend registered: direct
0.000030 libguestfs: launch: backend=libvirt
0.000031 libguestfs: launch: tmpdir=/tmp/libguestfsw18rBQ
0.000029 libguestfs: launch: umask=0002
0.000031 libguestfs: launch: euid=1000
0.000030 libguestfs: libvirt version = 1002012 (1.2.12)
[etc]
The timestamps are seconds (incrementally since the previous line).
libguestfs
プログラムから詳細なタイミングを取得するために
SystemTap (
stap(1))
を使用できます。
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);
}
/* すべての guestfs_* 関数の呼び出しを一覧表示します。 */
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_int_safe_strdup g=0x17a9de0 str=0x7f8a153bed5d
xxxx (+19): guestfs_int_safe_malloc g=0x17a9de0 nbytes=0x38
xxxx (+5): guestfs_int_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.
gdb
を使用してアプライアンスの
BIOS/カーネルに接続できます。実行することを理解している場合、ブートの逆行を診断するための有用な方法になりえます。
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 -
# 実際の QEMU バイナリーを指し示すようこれを設定してください。
qemu=/usr/bin/qemu-kvm
if [ "$1" != "-global" ]; then
# ヘルプの出力などを解析します。
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_HV=/path/to/qemu-wrapper guestfish -a /dev/null -v run
これは QEMU
の起動後に単に停止しています。他のウィンドウにおいて、gdb
を使用して QEMU
に接続します:
$ 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.
Sometimes performance regressions happen in other programs (eg. qemu, the
kernel) that cause problems for libguestfs.
In
https://github.com/libguestfs/libguestfs-analysis-tools
boot-benchmark/boot-benchmark-range.pl is a script which can be used to
benchmark libguestfs across a range of git commits in another project to find
out if any commit is causing a slowdown (or speedup).
To find out how to use this script, consult the manual:
./boot-benchmark/boot-benchmark-range.pl --man
supermin(1),
guestfish(1),
guestfs(3),
guestfs-examples(3),
guestfs-internals(1),
libguestfs-make-fixed-appliance(1),
stap(1),
qemu(1),
gdb(1),
http://libguestfs.org/.
Richard W.M. Jones ("rjones at redhat dot com")
Copyright (C) 2012-2020 Red Hat Inc.
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.