Re: [libvirt] OpenStack/libvirt CAT interface

On Tue, Jan 10, 2017 at 02:18:41PM -0200, Marcelo Tosatti wrote: > > There have been queries about the OpenStack interface > for CAT: > > http://bugzilla.redhat.com/show_bug.cgi?id=1299678 > > Comment 2 says: > Sahid Ferdjaoui 2016-01-19 10:58:48 EST > A spec will have to be addressed, after a first look this feature needs > some work in several components of Nova to maintain/schedule/consume > host's cache. I can work on that spec and implementation it when libvirt > will provides information about cache and feature to use it for guests. > > I could add a comment about parameters to resctrltool, but since > this depends on the libvirt interface, it would be good to know > what the libvirt interface exposes first. > > I believe it should be essentially similar to OpenStack's > "reserved_host_memory_mb": > > Set the reserved_host_memory_mb to reserve RAM for host > processes. For > the purposes of testing I am going to use the default of 512 MB: > reserved_host_memory_mb=512 > > But rather use: > > rdt_cat_cache_reservation=type=code/data/both,size=10mb,cacheid=2; > type=code/data/both,size=2mb,cacheid=1;... > > (per-vcpu). > > Where cache-id is optional. > > What is cache-id (from Documentation/x86/intel_rdt_ui.txt on recent > kernel sources): > Cache IDs > --------- > On current generation systems there is one L3 cache per socket and L2 > caches are generally just shared by the hyperthreads on a core, but this > isn't an architectural requirement. We could have multiple separate L3 > caches on a socket, multiple cores could share an L2 cache. So instead > of using "socket" or "core" to define the set of logical cpus sharing > a resource we use a "Cache ID". At a given cache level this will be a > unique number across the whole system (but it isn't guaranteed to be a > contiguous sequence, there may be gaps). To find the ID for each > logical > CPU look in /sys/devices/system/cpu/cpu*/cache/index*/id > > > WHAT THE USER NEEDS TO SPECIFY FOR VIRTUALIZATION (KVM-RT) > ========================================================== > > For virtualization the following scenario is desired, > on a given socket: > > * VM-A with VCPUs VM-A.vcpu-1, VM-A.vcpu-2. > * VM-B with VCPUs VM-B.vcpu-1, VM-B.vcpu-2. > > With one realtime workload on each vcpu-2. > > Assume VM-A.vcpu-2 on pcpu 3. > Assume VM-B.vcpu-2 on pcpu 5. > > Assume pcpus 0-5 on cacheid 0. > > We want VM-A.vcpu-2 to have a certain region of cache reserved, > and VM-B.vcpu-2 as well. vcpu-1 for both VMs can use the default group > (that is not have reserved L3 cache). > > This translates to the following resctrltool-style reservations: > > res.vm-a.vcpu-2 > > type=both,size=VM-A-RESSIZE,cache-id=0 > > res.vm-b.vcpu-2 > > type=both,size=VM-B-RESSIZE,cache-id=0 > > Which translate to the following in resctrlfs: > > res.vm-a.vcpu-2 > > type=both,size=VM-A-RESSIZE,cache-id=0 > type=both,size=default-size,cache-id=1 > ... > > res.vm-b.vcpu-2 > > type=both,size=VM-B-RESSIZE,cache-id=0 > type=both,size=default-size,cache-id=1 > ... > > Which is what we want, since the VCPUs are pinned. > > > res.vm-a.vcpu-1 and res.vm-b.vcpu-1 don't need to > be assigned to any reservation, which means they'll > remain on the default group. > > RESTRICTIONS TO THE SYNTAX ABOVE > ================================ > > Rules for the parameters: > * type=code must be paired with type=data entry. > > ABOUT THE LIST INTERFACE > ======================== > > About an interface for listing the reservations > of the system to OpenStack. > > I think that what OpenStack needs is to check, before > starting a guest on a given host, that there is sufficient > space available for the reservation. > > To do that, it can: > > 1) resctrltool list (the end of the output mentions > how much free space available there is), or > via resctrlfs directly (have to lock the filesystem, > read each directory, AND each schemata, and count > number of zero bits). > 2) Via libvirt > > Should fix resctrltool/API to list amount of contiguous free space > BTW. Elements of the libvirt CAT interface: