Re: [PATCH Libvirt v3 02/10] conf: Introduce XML for dirty limit configuration

On Tue, Aug 01, 2023 at 05:31:12PM +0800, ~hyman wrote: > From: Hyman Huang(黄勇) <yong.huang(a)smartx.com&gt; > > The upper limit (megabyte/s) of the dirty page rate configured > by the user can be tracked by the XML. To allow this, add the > following XML: > > <domain> > ... > <vcpu current='2'>3</vcpu> > <vcpus> > <vcpu id='0' hotpluggable='no' dirty_limit='10' order='1'.../> > <vcpu id='1' hotpluggable='yes' dirty_limit='10' order='2'.../> > </vcpus> > ... > > The "dirty_limit" attribute in "vcpu" sub-element within "vcpus" > element allows to set an upper limit for the individual vCPU. The > value can be set dynamically by limit-dirty-page-rate API. > > Note that the dirty limit feature is based on the dirty-ring > feature, so it requires dirty-ring size configuration in XML. > > Signed-off-by: Hyman Huang(黄勇) <yong.huang(a)smartx.com&gt; > --- > docs/formatdomain.rst | 7 ++++++- > src/conf/domain_conf.c | 26 ++++++++++++++++++++++++ > src/conf/domain_conf.h | 8 ++++++++ > src/conf/domain_validate.c | 33 +++++++++++++++++++++++++++++++ > src/conf/schemas/domaincommon.rng | 5 +++++ > 5 files changed, 78 insertions(+), 1 deletion(-) > > diff --git a/docs/formatdomain.rst b/docs/formatdomain.rst > index bc469e5f9f..337b7ec9cc 100644 > --- a/docs/formatdomain.rst > +++ b/docs/formatdomain.rst > @@ -715,6 +715,11 @@ CPU Allocation > be enabled and non-hotpluggable. On PPC64 along with it vCPUs that are in the > same core need to be enabled as well. All non-hotpluggable CPUs present at > boot need to be grouped after vCPU 0. :since:`Since 2.2.0 (QEMU only)` > + ``dirty_limit`` :since:`Since 9.7.0 (QEMU and KVM only)` > + The optional attribute ``dirty_limit`` allows to set an upper limit (MB/s) > + of the dirty page rate for the vCPU. User can change the upper limit value > + dynamically by using ``limit-dirty-page-rate`` API. Require ``dirty-ring`` > + size configured. What scenarios would you want to apply such a limit ?

Is there to admins for sensible values to use when setting this limit ?

What is the impact on the guest if it hits the limit ?

Is the snigle vCPU blocked for the remainder of some timeslice, or are all vCPUs blocked ?

Does it even make sense to control this with different values per-VCPU as opposed to a single value for the VM as a whole ?

With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| Thanks,