af39bd0d9a
QEMU 2.12 (commit 1221fe6f636754ab5f2c1c87caa77633e9123622) introduced a new setting called l2-cache-entry-size that allows making entries on the qcow2 L2 cache smaller than the cluster size. I have been performing several tests with different cluster and entry sizes and all of them show that reducing the entry size (aka L2 slice) consistently improves I/O performance, notably during random I/O (all tests done with sequential I/O show similar results). This is to be expected because loading and evicting an L2 slice is more expensive the larger the slice is. Here are some numbers on fully populated 40GB qcow2 images. The rightmost column represents the maximum L2 cache size in both cases. Cluster size = 64 KB |-------------+--------------+--------------+--------------| | | 1MB L2 cache | 3MB L2 cache | 5MB L2 cache | |-------------+--------------+--------------+--------------| | 4KB slices | 6545 IOPS | 12045 IOPS | 55680 IOPS | | 16KB slices | 5177 IOPS | 9798 IOPS | 56278 IOPS | | 64KB slices | 2718 IOPS | 5326 IOPS | 57355 IOPS | |-------------+--------------+--------------+--------------| Cluster size = 256 KB |--------------+----------------+--------------+-----------------| | | 512KB L2 cache | 1MB L2 cache | 1280KB L2 cache | |--------------+----------------+--------------+-----------------| | 4KB slices | 8539 IOPS | 21071 IOPS | 55417 IOPS | | 64KB slices | 3598 IOPS | 9772 IOPS | 57687 IOPS | | 256KB slices | 1415 IOPS | 4120 IOPS | 58001 IOPS | |--------------+----------------+--------------+-----------------| As can be seen in the numbers, the only exception to the rule is when the cache is large enough to hold all L2 tables. This is also to be expected because in this case no cache entry is ever evicted so reducing its size doesn't bring any benefit. This patch sets the default L2 cache entry size to 4KB except when the cache is large enough for the whole disk. Signed-off-by: Alberto Garcia <berto@igalia.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
QEMU README
===========
QEMU is a generic and open source machine & userspace emulator and
virtualizer.
QEMU is capable of emulating a complete machine in software without any
need for hardware virtualization support. By using dynamic translation,
it achieves very good performance. QEMU can also integrate with the Xen
and KVM hypervisors to provide emulated hardware while allowing the
hypervisor to manage the CPU. With hypervisor support, QEMU can achieve
near native performance for CPUs. When QEMU emulates CPUs directly it is
capable of running operating systems made for one machine (e.g. an ARMv7
board) on a different machine (e.g. an x86_64 PC board).
QEMU is also capable of providing userspace API virtualization for Linux
and BSD kernel interfaces. This allows binaries compiled against one
architecture ABI (e.g. the Linux PPC64 ABI) to be run on a host using a
different architecture ABI (e.g. the Linux x86_64 ABI). This does not
involve any hardware emulation, simply CPU and syscall emulation.
QEMU aims to fit into a variety of use cases. It can be invoked directly
by users wishing to have full control over its behaviour and settings.
It also aims to facilitate integration into higher level management
layers, by providing a stable command line interface and monitor API.
It is commonly invoked indirectly via the libvirt library when using
open source applications such as oVirt, OpenStack and virt-manager.
QEMU as a whole is released under the GNU General Public License,
version 2. For full licensing details, consult the LICENSE file.
Building
========
QEMU is multi-platform software intended to be buildable on all modern
Linux platforms, OS-X, Win32 (via the Mingw64 toolchain) and a variety
of other UNIX targets. The simple steps to build QEMU are:
mkdir build
cd build
../configure
make
Additional information can also be found online via the QEMU website:
https://qemu.org/Hosts/Linux
https://qemu.org/Hosts/Mac
https://qemu.org/Hosts/W32
Submitting patches
==================
The QEMU source code is maintained under the GIT version control system.
git clone https://git.qemu.org/git/qemu.git
When submitting patches, one common approach is to use 'git
format-patch' and/or 'git send-email' to format & send the mail to the
qemu-devel@nongnu.org mailing list. All patches submitted must contain
a 'Signed-off-by' line from the author. Patches should follow the
guidelines set out in the HACKING and CODING_STYLE files.
Additional information on submitting patches can be found online via
the QEMU website
https://qemu.org/Contribute/SubmitAPatch
https://qemu.org/Contribute/TrivialPatches
The QEMU website is also maintained under source control.
git clone https://git.qemu.org/git/qemu-web.git
https://www.qemu.org/2017/02/04/the-new-qemu-website-is-up/
A 'git-publish' utility was created to make above process less
cumbersome, and is highly recommended for making regular contributions,
or even just for sending consecutive patch series revisions. It also
requires a working 'git send-email' setup, and by default doesn't
automate everything, so you may want to go through the above steps
manually for once.
For installation instructions, please go to
https://github.com/stefanha/git-publish
The workflow with 'git-publish' is:
$ git checkout master -b my-feature
$ # work on new commits, add your 'Signed-off-by' lines to each
$ git publish
Your patch series will be sent and tagged as my-feature-v1 if you need to refer
back to it in the future.
Sending v2:
$ git checkout my-feature # same topic branch
$ # making changes to the commits (using 'git rebase', for example)
$ git publish
Your patch series will be sent with 'v2' tag in the subject and the git tip
will be tagged as my-feature-v2.
Bug reporting
=============
The QEMU project uses Launchpad as its primary upstream bug tracker. Bugs
found when running code built from QEMU git or upstream released sources
should be reported via:
https://bugs.launchpad.net/qemu/
If using QEMU via an operating system vendor pre-built binary package, it
is preferable to report bugs to the vendor's own bug tracker first. If
the bug is also known to affect latest upstream code, it can also be
reported via launchpad.
For additional information on bug reporting consult:
https://qemu.org/Contribute/ReportABug
Contact
=======
The QEMU community can be contacted in a number of ways, with the two
main methods being email and IRC
- qemu-devel@nongnu.org
https://lists.nongnu.org/mailman/listinfo/qemu-devel
- #qemu on irc.oftc.net
Information on additional methods of contacting the community can be
found online via the QEMU website:
https://qemu.org/Contribute/StartHere
-- End