Hi everybody, a while ago there was a discussion about changing the data that is returned in the <topology> sub-element: <capabilities> <host> <cpu> <arch>x86_64</arch> <model>SandyBridge</model> <vendor>Intel</vendor> <topology sockets='1' cores='2' threads='2'/> The data provided here is as of today taken from the nodeinfo detection code and thus is really wrong when the fallback mechanisms are used. To get a useful count, the user has to multiply the data by the number of NUMA nodes in the host. With the fallback detection code used for nodeinfo the NUMA node count used to get the CPU count should be 1 instead of the actual number. As Jiri proposed, I think we should change this output to separate detection code that will not take into account NUMA nodes for this output and will rather provide data as the "lspci" command does. This change will make the data provided by the element standalone and also usable in guest XMLs to mirror host's topology.

From a historical compatibility pov I don't think we should be changing

On 01/16/13 19:11, Daniel P. Berrange wrote: >On Wed, Jan 16, 2013 at 05:28:57PM +0100, Peter Krempa wrote: >>Hi everybody, >> >>a while ago there was a discussion about changing the data that is >>returned in the <topology> sub-element: >> >><capabilities> >> <host> >> <cpu> >> <arch>x86_64</arch> >> <model>SandyBridge</model> >> <vendor>Intel</vendor> >> <topology sockets='1' cores='2' threads='2'/> >> >> >>The data provided here is as of today taken from the nodeinfo >>detection code and thus is really wrong when the fallback mechanisms >>are used. >> >>To get a useful count, the user has to multiply the data by the >>number of NUMA nodes in the host. With the fallback detection code >>used for nodeinfo the NUMA node count used to get the CPU count >>should be 1 instead of the actual number. >> >>As Jiri proposed, I think we should change this output to separate >>detection code that will not take into account NUMA nodes for this >>output and will rather provide data as the "lspci" command does. >> >>This change will make the data provided by the element standalone >>and also usable in guest XMLs to mirror host's topology. > >Well there are 2 parts which need to be considered here. What do we report >in the host capabilities, and how do you configure guest XML. > > From a historical compatibility pov I don't think we should be changing >the host capabilities at all. Simply document that 'sockets' is treated >as sockets-per-node everywhere, and that it is wrong in the case of >machines where an socket can internally have multiple NUMA nodes. I'm too somewhat concerned about changing this output due to historic reasons. > >Apps should be using the separate NUMA <topology> data in the capabilities >instead of the CPU <topology> data, to get accurate CPU counts. From the NUMA <topology> the management apps can't tell if the CPU is a core or a thread. For example oVirt/VDSM bases the decisions on this information.

----- Original Message ----- From: Daniel P. Berrange <berrange(a)redhat.com&gt; To: Peter Krempa <pkrempa(a)redhat.com&gt; Cc: Jiri Denemark <jdenemar(a)redhat.com&gt;, Amador Pahim <apahim(a)redhat.com&gt;, libvirt-list(a)redhat.com, dougsland(a)redhat.com Sent: Wed, 16 Jan 2013 13:39:28 -0500 (EST) Subject: Re: [libvirt] [RFC] Data in the <topology> element in the capabilities XML On Wed, Jan 16, 2013 at 07:31:02PM +0100, Peter Krempa wrote: > On 01/16/13 19:11, Daniel P. Berrange wrote: > >On Wed, Jan 16, 2013 at 05:28:57PM +0100, Peter Krempa wrote: > >>Hi everybody, > >> > >>a while ago there was a discussion about changing the data that is > >>returned in the <topology> sub-element: > >> > >><capabilities> > >> <host> > >> <cpu> > >> <arch>x86_64</arch> > >> <model>SandyBridge</model> > >> <vendor>Intel</vendor> > >> <topology sockets='1' cores='2' threads='2'/> > >> > >> > >>The data provided here is as of today taken from the nodeinfo > >>detection code and thus is really wrong when the fallback mechanisms > >>are used. > >> > >>To get a useful count, the user has to multiply the data by the > >>number of NUMA nodes in the host. With the fallback detection code > >>used for nodeinfo the NUMA node count used to get the CPU count > >>should be 1 instead of the actual number. > >> > >>As Jiri proposed, I think we should change this output to separate > >>detection code that will not take into account NUMA nodes for this > >>output and will rather provide data as the "lspci" command does. > >> > >>This change will make the data provided by the element standalone > >>and also usable in guest XMLs to mirror host's topology. > > > >Well there are 2 parts which need to be considered here. What do we report > >in the host capabilities, and how do you configure guest XML. > > > > From a historical compatibility pov I don't think we should be changing > >the host capabilities at all. Simply document that 'sockets' is treated > >as sockets-per-node everywhere, and that it is wrong in the case of > >machines where an socket can internally have multiple NUMA nodes. > > I'm too somewhat concerned about changing this output due to > historic reasons. > > > >Apps should be using the separate NUMA <topology> data in the capabilities > >instead of the CPU <topology> data, to get accurate CPU counts. > > From the NUMA <topology> the management apps can't tell if the CPU > is a core or a thread. For example oVirt/VDSM bases the decisions on > this information. Then, we should add information to the NUMA topology XML to indicate which of the child <cpu> elements are sibling cores or threads. Perhaps add a 'socket_id' + 'core_id' attribute to every <cpu>.

In this case, we will also need to add the thread siblings and perhaps even core siblings information to allow reliable detection.

On 01/16/2013 04:30 PM, Daniel P. Berrange wrote: >On Wed, Jan 16, 2013 at 02:15:37PM -0500, Peter Krempa wrote: >>----- Original Message ----- >>From: Daniel P. Berrange <berrange(a)redhat.com&gt; >>To: Peter Krempa <pkrempa(a)redhat.com&gt; >>Cc: Jiri Denemark <jdenemar(a)redhat.com&gt;, Amador Pahim <apahim(a)redhat.com&gt;, libvirt-list(a)redhat.com, dougsland(a)redhat.com >>Sent: Wed, 16 Jan 2013 13:39:28 -0500 (EST) >>Subject: Re: [libvirt] [RFC] Data in the <topology> element in the capabilities XML >> >>On Wed, Jan 16, 2013 at 07:31:02PM +0100, Peter Krempa wrote: >>>On 01/16/13 19:11, Daniel P. Berrange wrote: >>>>On Wed, Jan 16, 2013 at 05:28:57PM +0100, Peter Krempa wrote: >>>>>Hi everybody, >>>>> >>>>>a while ago there was a discussion about changing the data that is >>>>>returned in the <topology> sub-element: >>>>> >>>>><capabilities> >>>>><host> >>>>><cpu> >>>>><arch>x86_64</arch> >>>>><model>SandyBridge</model> >>>>><vendor>Intel</vendor> >>>>><topology sockets='1' cores='2' threads='2'/> >>>>> >>>>> >>>>>The data provided here is as of today taken from the nodeinfo >>>>>detection code and thus is really wrong when the fallback mechanisms >>>>>are used. >>>>> >>>>>To get a useful count, the user has to multiply the data by the >>>>>number of NUMA nodes in the host. With the fallback detection code >>>>>used for nodeinfo the NUMA node count used to get the CPU count >>>>>should be 1 instead of the actual number. >>>>> >>>>>As Jiri proposed, I think we should change this output to separate >>>>>detection code that will not take into account NUMA nodes for this >>>>>output and will rather provide data as the "lspci" command does. >>>>> >>>>>This change will make the data provided by the element standalone >>>>>and also usable in guest XMLs to mirror host's topology. >>>>Well there are 2 parts which need to be considered here. What do we report >>>>in the host capabilities, and how do you configure guest XML. >>>> >>>> From a historical compatibility pov I don't think we should be changing >>>>the host capabilities at all. Simply document that 'sockets' is treated >>>>as sockets-per-node everywhere, and that it is wrong in the case of >>>>machines where an socket can internally have multiple NUMA nodes. >>>I'm too somewhat concerned about changing this output due to >>>historic reasons. >>>>Apps should be using the separate NUMA <topology> data in the capabilities >>>>instead of the CPU <topology> data, to get accurate CPU counts. >>> From the NUMA <topology> the management apps can't tell if the CPU >>>is a core or a thread. For example oVirt/VDSM bases the decisions on >>>this information. >>Then, we should add information to the NUMA topology XML to indicate >>which of the child <cpu> elements are sibling cores or threads. >> >>Perhaps add a 'socket_id' + 'core_id' attribute to every <cpu>. > >>In this case, we will also need to add the thread siblings and >>perhaps even core siblings information to allow reliable detection. >The combination fo core_id/socket_id lets you determine that. If two >core have the same socket_id then they are cores or threads within the >same socket. If two <cpu> have the same socket_id & core_id then they >are threads within the same core. Not true to AMD Magny-Cours 6100 series, where different cores can share the same physical_id and core_id. And they are not threads. This processors has two numa nodes inside the same "package" (aka socket) and they shares the same core ID set. Annoying.

On 01/16/13 21:24, Daniel P. Berrange wrote: >On Wed, Jan 16, 2013 at 05:06:21PM -0300, Amador Pahim wrote: >>On 01/16/2013 04:30 PM, Daniel P. Berrange wrote: >>>On Wed, Jan 16, 2013 at 02:15:37PM -0500, Peter Krempa wrote: >>>>----- Original Message ----- >>>>From: Daniel P. Berrange <berrange(a)redhat.com&gt; >>>>To: Peter Krempa <pkrempa(a)redhat.com&gt; >>>>Cc: Jiri Denemark <jdenemar(a)redhat.com&gt;, Amador Pahim <apahim(a)redhat.com&gt;, libvirt-list(a)redhat.com, dougsland(a)redhat.com >>>>Sent: Wed, 16 Jan 2013 13:39:28 -0500 (EST) >>>>Subject: Re: [libvirt] [RFC] Data in the <topology> element in the capabilities XML >>>> >>>>On Wed, Jan 16, 2013 at 07:31:02PM +0100, Peter Krempa wrote: >>>>>On 01/16/13 19:11, Daniel P. Berrange wrote: >>>>>>On Wed, Jan 16, 2013 at 05:28:57PM +0100, Peter Krempa wrote: >>>>>>>Hi everybody, >>>>>>> >>>>>>>a while ago there was a discussion about changing the data that is >>>>>>>returned in the <topology> sub-element: >>>>>>> >>>>>>><capabilities> >>>>>>><host> >>>>>>><cpu> >>>>>>><arch>x86_64</arch> >>>>>>><model>SandyBridge</model> >>>>>>><vendor>Intel</vendor> >>>>>>><topology sockets='1' cores='2' threads='2'/> >>>>>>> >>>>>>> >>>>>>>The data provided here is as of today taken from the nodeinfo >>>>>>>detection code and thus is really wrong when the fallback mechanisms >>>>>>>are used. >>>>>>> >>>>>>>To get a useful count, the user has to multiply the data by the >>>>>>>number of NUMA nodes in the host. With the fallback detection code >>>>>>>used for nodeinfo the NUMA node count used to get the CPU count >>>>>>>should be 1 instead of the actual number. >>>>>>> >>>>>>>As Jiri proposed, I think we should change this output to separate >>>>>>>detection code that will not take into account NUMA nodes for this >>>>>>>output and will rather provide data as the "lspci" command does. >>>>>>> >>>>>>>This change will make the data provided by the element standalone >>>>>>>and also usable in guest XMLs to mirror host's topology. >>>>>>Well there are 2 parts which need to be considered here. What do we report >>>>>>in the host capabilities, and how do you configure guest XML. >>>>>> >>>>>> From a historical compatibility pov I don't think we should be changing >>>>>>the host capabilities at all. Simply document that 'sockets' is treated >>>>>>as sockets-per-node everywhere, and that it is wrong in the case of >>>>>>machines where an socket can internally have multiple NUMA nodes. >>>>>I'm too somewhat concerned about changing this output due to >>>>>historic reasons. >>>>>>Apps should be using the separate NUMA <topology> data in the capabilities >>>>>>instead of the CPU <topology> data, to get accurate CPU counts. >>>>> From the NUMA <topology> the management apps can't tell if the CPU >>>>>is a core or a thread. For example oVirt/VDSM bases the decisions on >>>>>this information. >>>>Then, we should add information to the NUMA topology XML to indicate >>>>which of the child <cpu> elements are sibling cores or threads. >>>> >>>>Perhaps add a 'socket_id' + 'core_id' attribute to every <cpu>. >>> >>>>In this case, we will also need to add the thread siblings and >>>>perhaps even core siblings information to allow reliable detection. >>>The combination fo core_id/socket_id lets you determine that. If two >>>core have the same socket_id then they are cores or threads within the >>>same socket. If two <cpu> have the same socket_id & core_id then they >>>are threads within the same core. >> >>Not true to AMD Magny-Cours 6100 series, where different cores can >>share the same physical_id and core_id. And they are not threads. >>This processors has two numa nodes inside the same "package" (aka >>socket) and they shares the same core ID set. Annoying. > >I don't believe there's a problem with that. This example XML >shows a machine with 4 NUMA nodes, 2 sockets each containing >2 cores, and 2 threads, giving 16 logical CPUs > > <topology> > <cells num='4'> > <cell id='0'> > <cpus num='4'> > <cpu id='0' socket_id='0' core_id='0'/> > <cpu id='1' socket_id='0' core_id='0'/> > <cpu id='2' socket_id='0' core_id='1'/> > <cpu id='3' socket_id='0' core_id='1'/> > </cpus> > </cell> > <cell id='1'> > <cpus num='2'> > <cpu id='4' socket_id='0' core_id='0'/> > <cpu id='5' socket_id='0' core_id='0'/> > <cpu id='6' socket_id='0' core_id='1'/> > <cpu id='7' socket_id='0' core_id='1'/> > </cpus> > </cell> > <cell id='2'> > <cpus num='2'> > <cpu id='8' socket_id='1' core_id='0'/> > <cpu id='9' socket_id='1' core_id='0'/> > <cpu id='10' socket_id='1' core_id='1'/> > <cpu id='11' socket_id='1' core_id='1'/> > </cpus> > </cell> > <cell id='3'> > <cpus num='2'> > <cpu id='12' socket_id='1' core_id='0'/> > <cpu id='13' socket_id='1' core_id='0'/> > <cpu id='14' socket_id='1' core_id='1'/> > <cpu id='15' socket_id='1' core_id='1'/> > </cpus> > </cell> > </cells> > </topology> > >I believe there's enough info there to determine all the co-location >aspects of all the sockets/core/threads involved. Well not for all machines in the wild out there. This is a very similar approach that libvirt uses now to detect the topology and it is not enough to detect threads on AMD Bulldozer as the cpus corresponding to the threads have different core_id's (they are also considered as cores from the perspective of the kernel). This is unfortunate for the virtualization management tools as oVirt that still consider the AMD Bulldozer "module" as a 1 core with two threads, even if it registers as two cores. For AMD Bulldozer to be detected correctly, we would need to expose the thread_id's along with thread siblings information to determine the two threads belonging together.

On 01/17/13 10:36, Daniel P. Berrange wrote: >> >>Well not for all machines in the wild out there. This is a very >>similar approach that libvirt uses now to detect the topology and it >>is not enough to detect threads on AMD Bulldozer as the cpus >>corresponding to the threads have different core_id's (they are also >>considered as cores from the perspective of the kernel). This is >>unfortunate for the virtualization management tools as oVirt that >>still consider the AMD Bulldozer "module" as a 1 core with two >>threads, even if it registers as two cores. >> >>For AMD Bulldozer to be detected correctly, we would need to expose >>the thread_id's along with thread siblings information to determine >>the two threads belonging together. > >NB, the socket_id / core_id values in the above XML are *not* intended >to be anyway related to similarly named values in /proc/cpuinfo. They >are values libvirt assigns to show the topology accurately. Hm, in that case I'm not sure if it's worth bothering with detecting the topology, then (possibly) making up the data provided in the XML. The management applications then will have to calculate the topology from our data again just to know the topology. For other possible uses of the data the management app would still need to re-detect it on their own if they need (for some reason) the actual data.

Also, even with non-real data in these fields here it won't enable us to reflect the topology of AMD Bulldozer reliably. We would have to choose whether we'd like to report the modules as cores or threads. And each of these choices will possibly make someone complain that they don't like the choice.

On 01/17/13 15:48, Daniel P. Berrange wrote: >On Thu, Jan 17, 2013 at 03:41:44PM +0100, Peter Krempa wrote: >>On 01/17/13 10:36, Daniel P. Berrange wrote: >>>> >>>>Well not for all machines in the wild out there. This is a very >>>>similar approach that libvirt uses now to detect the topology and it >>>>is not enough to detect threads on AMD Bulldozer as the cpus >>>>corresponding to the threads have different core_id's (they are also >>>>considered as cores from the perspective of the kernel). This is >>>>unfortunate for the virtualization management tools as oVirt that >>>>still consider the AMD Bulldozer "module" as a 1 core with two >>>>threads, even if it registers as two cores. >>>> >>>>For AMD Bulldozer to be detected correctly, we would need to expose >>>>the thread_id's along with thread siblings information to determine >>>>the two threads belonging together. >>> >>>NB, the socket_id / core_id values in the above XML are *not* intended >>>to be anyway related to similarly named values in /proc/cpuinfo. They >>>are values libvirt assigns to show the topology accurately. >> >>Hm, in that case I'm not sure if it's worth bothering with detecting >>the topology, then (possibly) making up the data provided in the >>XML. The management applications then will have to calculate the >>topology from our data again just to know the topology. For other >>possible uses of the data the management app would still need to >>re-detect it on their own if they need (for some reason) the actual >>data. > >The point is that libvirt is intended to provide a representation >of the data that is independant of the underlying technology. Apps >using libvirt can't expect to read /proc/cpuinfo directly and then >expect libvirt to match that exactly. They should be using the >libvirt APIs/XML for this exclusively & if there is something missing >which prevents them doing this, then we need to add it. Okay, this is fair enough and might work even for non-symetric multiprocessor systems. > >>Also, even with non-real data in these fields here it won't enable >>us to reflect the topology of AMD Bulldozer reliably. We would have >>to choose whether we'd like to report the modules as cores or >>threads. And each of these choices will possibly make someone >>complain that they don't like the choice. > >I think you're creating a problem where none exists - there is a clear >difference between what is a hyperthread vs what is a core, so there >is no ambiguity in what we choose to use. We must simply pick the one >that is right. > I beg to differ: "This technology is informally called CMT (Clustered Multi-Thread) and formally called "module" by the AMD's marketing service, but whose real name is "Clustered Integer" Core Technology. In terms of hardware complexity and functionality, this "module" is midway between a true dual-core processor and its integer power (each thread having a fully independent integer core) and a single core processor that has the SMT ability, which can create a dual threads processor but with the power of one (each thread shares the resources of the core with the other thread)." (source: http://en.wikipedia.org/wiki/Bulldozer_(microarchitecture)#BULLDOZER_Core... ) oVirt/vdsm considers the "Module" as a core with two threads, where others consider it more as separate cores. The performance depends on the type of task that is being run on the module.

On 01/17/13 16:43, Daniel P. Berrange wrote: VDSM uses the <topology> element (subelement of cpu) in the capabilities and multiplies the numbers by the number of numa nodes. As the data there is taken from nodeinfo, this breaks on some systems.