
=head1 名前

guestfs-performance - engineering libguestfs for greatest performance

=head1 説明

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.

=head1 BASELINE MEASUREMENTS

Before making changes to how you use libguestfs, take baseline measurements.

=head2 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.

I<Side note for developers:> There is a program called F<boot-benchmark> in
L<https://github.com/libguestfs/libguestfs-analysis-tools> which does the
same thing, but performs multiple runs and prints the mean and standard
deviation.

=head3 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
F</var/tmp/.guestfs-*>).  Subsequent runs should reuse the cached appliance.

=head3 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
L</TROUBLESHOOTING POOR PERFORMANCE> below.

=head2 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.

=head3 説明

This command starts up the libguestfs appliance on the named disk image or
libvirt guest, performs libguestfs inspection on it (see
L<guestfs(3)/INSPECTION>), 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 F</var/tmp/.guestfs-*>).  Subsequent runs should reuse the
cached appliance.

=head3 期待される結果

You should expect times which are E<le> 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 E<le> 10 seconds).

=head1 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 F</var/tmp/.guestfs-*>, unless you have set C<$TMPDIR> or
C<$LIBGUESTFS_CACHEDIR> in which case it will be under that temporary
directory.

For more information about how the appliance is constructed, see
L<supermin(1)/SUPERMIN APPLIANCES>.

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 C<yum> or C<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:

=over 4

=item *

The process of building (or rebuilding) the cached appliance is slow, and
you can avoid this happening by using a fixed appliance (see below).

=item *

If not using a fixed appliance, be aware that updating software on the host
will cause a one time rebuild of the appliance.

=item *

F</var/tmp> (or C<$TMPDIR>, C<$LIBGUESTFS_CACHEDIR>) should be on a fast
disk, and have plenty of space for the appliance.

=back

=head1 固定アプライアンスの使用法

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 C<E<lt>directoryE<gt>> with the name of a directory where the
appliance will be stored (normally you would name a subdirectory, for
example: F</usr/local/lib/guestfs/appliance> or F</dev/shm/appliance>).

Then set C<$LIBGUESTFS_PATH> (and ensure this environment variable is set in
your libguestfs program), or modify your program so it calls
C<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 L<libguestfs-make-fixed-appliance(1)> を参照してください)。

=head2 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 F</dev/shm>).  However there are two points to consider:

=over 4

=item 1.

Using a fixed appliance stops libguestfs from ever rebuilding the appliance,
meaning that libguestfs will have more predictable start-up times.

=item 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.

=back

=head1 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 C<guestfs_launch> at most once per target virtual machine or
disk image.

Instead of using a separate instance of L<guestfish(1)> to make a series of
changes to the same guest, use a single instance of guestfish and/or use the
guestfish I<--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:

=over 4

=item 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.

=item 2.

There is a hard limit to the number of disks you can add to a single
appliance.  Call L<guestfs(3)/guestfs_max_disks> to get this limit.  For
further information see L<guestfs(3)/LIMITS>.

=item 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 C<guestfs_lvm_set_filter>).

=back

L<virt-df(1)> adds multiple disks by default, so the source code for this
program would be a good place to start.

=head1 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 (C<guestfs_inspect_os>) is required, auxiliary
inspection operations may be avoided:

=over 4

=item *

Mounting disks is only necessary to get further filesystem information.

=item *

Listing applications (C<guestfs_inspect_list_applications>) is an expensive
operation on Linux, but almost free on Windows.

=item *

Generating a guest icon (C<guestfs_inspect_get_icon>) is cheap on Linux but
expensive on Windows.

=back

=head1 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:

L<http://rwmj.wordpress.com/2013/02/25/multiple-libguestfs-appliances-in-parallel-part-1/>
L<http://rwmj.wordpress.com/2013/02/25/multiple-libguestfs-appliances-in-parallel-part-2/>
L<http://rwmj.wordpress.com/2013/02/25/multiple-libguestfs-appliances-in-parallel-part-3/>
L<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);
 }

=head1 USING USER-MODE LINUX

Since libguestfs 1.24, it has been possible to use the User-Mode Linux (uml)
backend instead of KVM (see L<guestfs(3)/USER-MODE LINUX BACKEND>).  This
section makes some general remarks about this backend, but it is B<highly
advisable> to measure your own workload under UML rather than trusting
comments or intuition.

=over 4

=item *

UML usually performs the same or slightly slower than KVM, on baremetal.

=item *

However UML often performs the same under virtualization as it does on
baremetal, whereas KVM can run much slower under virtualization (since
hardware virt acceleration is not available).

=item *

Upload and download is as much as 10 times slower on UML than KVM.
Libguestfs sends this data over the UML emulated serial port, which is far
less efficient than KVM’s virtio-serial.

=item *

UML lacks some features (eg. qcow2 support), so it may not be applicable at
all.

=back

For some actual figures, see:
L<http://rwmj.wordpress.com/2013/08/14/performance-of-user-mode-linux-as-a-libguestfs-backend/#content>

=head1 性能劣化のトラブルシューティング

=head2 Ensure hardware virtualization is available

Use F</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。

=head2 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 F</dev/kvm> and
most distributions now do this.

=head2 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).

=head2 Xen dom0

In Xen, dom0 is a virtual machine, and so hardware virtualization is not
available.

=head2 Use libguestfs E<ge> 1.34 and qemu E<ge> 2.7

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.

=head1 DETAILED ANALYSIS

=head2 Boot analysis

In L<https://github.com/libguestfs/libguestfs-analysis-tools> is a program
called C<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.

=head2 Detailed timings using ts

Use the L<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).

=head2 Detailed timings using SystemTap

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 (L<stap(1)>) を使用できます。

Save the following script as F<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.

=head2 Detailed debugging using gdb

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 C<-S> and C<-s>
options.  These options cause qemu to pause at boot and allow you to attach
a debugger.  Read L<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
L<guestfs(3)/QEMU WRAPPERS> 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<^C> to
interrupt the boot and C<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.

=head1 PERFORMANCE REGRESSIONS IN OTHER PROGRAMS

Sometimes performance regressions happen in other programs (eg. qemu, the
kernel) that cause problems for libguestfs.

In L<https://github.com/libguestfs/libguestfs-analysis-tools>
F<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

=head1 関連項目

L<supermin(1)>, L<guestfish(1)>, L<guestfs(3)>, L<guestfs-examples(3)>,
L<guestfs-internals(1)>, L<libguestfs-make-fixed-appliance(1)>, L<stap(1)>,
L<qemu(1)>, L<gdb(1)>, L<http://libguestfs.org/>.

=head1 著者

Richard W.M. Jones (C<rjones at redhat dot com>)

=head1 COPYRIGHT

Copyright (C) 2012-2020 Red Hat Inc.

