The coroutine pool implementation can hit the Linux vm.max_map_count
limit, causing QEMU to abort with "failed to allocate memory for stack"
or "failed to set up stack guard page" during coroutine creation.
This happens because per-thread pools can grow to tens of thousands of
coroutines. Each coroutine causes 2 virtual memory areas to be created.
Eventually vm.max_map_count is reached and memory-related syscalls fail.
The per-thread pool sizes are non-uniform and depend on past coroutine
usage in each thread, so it's possible for one thread to have a large
pool while another thread's pool is empty.
Switch to a new coroutine pool implementation with a global pool that
grows to a maximum number of coroutines and per-thread local pools that
are capped at hardcoded small number of coroutines.
This approach does not leave large numbers of coroutines pooled in a
thread that may not use them again. In order to perform well it
amortizes the cost of global pool accesses by working in batches of
coroutines instead of individual coroutines.
The global pool is a list. Threads donate batches of coroutines to when
they have too many and take batches from when they have too few:
.-----------------------------------.
| Batch 1 | Batch 2 | Batch 3 | ... | global_pool
`-----------------------------------'
Each thread has up to 2 batches of coroutines:
.-------------------.
| Batch 1 | Batch 2 | per-thread local_pool (maximum 2 batches)
`-------------------'
The goal of this change is to reduce the excessive number of pooled
coroutines that cause QEMU to abort when vm.max_map_count is reached
without losing the performance of an adequately sized coroutine pool.
Here are virtio-blk disk I/O benchmark results:
RW BLKSIZE IODEPTH OLD NEW CHANGE
randread 4k 1 113725 117451 +3.3%
randread 4k 8 192968 198510 +2.9%
randread 4k 16 207138 209429 +1.1%
randread 4k 32 212399 215145 +1.3%
randread 4k 64 218319 221277 +1.4%
randread 128k 1 17587 17535 -0.3%
randread 128k 8 17614 17616 +0.0%
randread 128k 16 17608 17609 +0.0%
randread 128k 32 17552 17553 +0.0%
randread 128k 64 17484 17484 +0.0%
See files/{fio.sh,test.xml.j2} for the benchmark configuration:
https://gitlab.com/stefanha/virt-playbooks/-/tree/coroutine-pool-fix-sizing
Buglink: https://issues.redhat.com/browse/RHEL-28947
Reported-by: Sanjay Rao <srao@redhat.com>
Reported-by: Boaz Ben Shabat <bbenshab@redhat.com>
Reported-by: Joe Mario <jmario@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20240318183429.1039340-1-stefanha@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.
Documentation
=============
Documentation can be found hosted online at
`<https://www.qemu.org/documentation/>`_. The documentation for the
current development version that is available at
`<https://www.qemu.org/docs/master/>`_ is generated from the ``docs/``
folder in the source tree, and is built by `Sphinx
<https://www.sphinx-doc.org/en/master/>`_.
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:
.. code-block:: shell
mkdir build
cd build
../configure
make
Additional information can also be found online via the QEMU website:
* `<https://wiki.qemu.org/Hosts/Linux>`_
* `<https://wiki.qemu.org/Hosts/Mac>`_
* `<https://wiki.qemu.org/Hosts/W32>`_
Submitting patches
==================
The QEMU source code is maintained under the GIT version control system.
.. code-block:: shell
git clone https://gitlab.com/qemu-project/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 `style section
<https://www.qemu.org/docs/master/devel/style.html>`_ of
the Developers Guide.
Additional information on submitting patches can be found online via
the QEMU website
* `<https://wiki.qemu.org/Contribute/SubmitAPatch>`_
* `<https://wiki.qemu.org/Contribute/TrivialPatches>`_
The QEMU website is also maintained under source control.
.. code-block:: shell
git clone https://gitlab.com/qemu-project/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:
.. code-block:: shell
$ 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:
.. code-block:: shell
$ 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 GitLab issues to track bugs. Bugs
found when running code built from QEMU git or upstream released sources
should be reported via:
* `<https://gitlab.com/qemu-project/qemu/-/issues>`_
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 GitLab.
For additional information on bug reporting consult:
* `<https://wiki.qemu.org/Contribute/ReportABug>`_
ChangeLog
=========
For version history and release notes, please visit
`<https://wiki.qemu.org/ChangeLog/>`_ or look at the git history for
more detailed information.
Contact
=======
The QEMU community can be contacted in a number of ways, with the two
main methods being email and IRC
* `<mailto: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://wiki.qemu.org/Contribute/StartHere>`_
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