On 14.10.22 15:47, David Hildenbrand wrote:
This is a follow-up on "util: NUMA aware memory preallocation" [1] by Michal.Setting the CPU affinity of threads from inside QEMU usually isn't easily possible, because we don't want QEMU -- once started and running guest code -- to be able to mess up the system. QEMU disallows relevant syscalls using seccomp, such that any such invocation will fail. Especially for memory preallocation in memory backends, the CPU affinity can significantly increase guest startup time, for example, when running large VMs backed by huge/gigantic pages, because of NUMA effects. For NUMA-aware preallocation, we have to set the CPU affinity, however: (1) Once preallocation threads are created during preallocation, management tools cannot intercept anymore to change the affinity. These threads are created automatically on demand. (2) QEMU cannot easily set the CPU affinity itself. (3) The CPU affinity derived from the NUMA bindings of the memory backend might not necessarily be exactly the CPUs we actually want to use (e.g., CPU-less NUMA nodes, CPUs that are pinned/used for other VMs). There is an easy "workaround". If we have a thread with the right CPU affinity, we can simply create new threads on demand via that prepared context. So, all we have to do is setup and create such a context ahead of time, to then configure preallocation to create new threads via that environment. So, let's introduce a user-creatable "thread-context" object that essentially consists of a context thread used to create new threads. QEMU can either try setting the CPU affinity itself ("cpu-affinity", "node-affinity" property), or upper layers can extract the thread id ("thread-id" property) to configure it externally. Make memory-backends consume a thread-context object (via the "prealloc-context" property) and use it when preallocating to create new threads with the desired CPU affinity. Further, to make it easier to use, allow creation of "thread-context" objects, including setting the CPU affinity directly from QEMU, before enabling the sandbox option. Quick test on a system with 2 NUMA nodes: Without CPU affinity: time qemu-system-x86_64 \ -object memory-backend-memfd,id=md1,hugetlb=on,hugetlbsize=2M,size=64G,prealloc-threads=12,prealloc=on,host-nodes=0,policy=bind \ -nographic -monitor stdio real 0m5.383s real 0m3.499s real 0m5.129s real 0m4.232s real 0m5.220s real 0m4.288s real 0m3.582s real 0m4.305s real 0m5.421s real 0m4.502s -> It heavily depends on the scheduler CPU selection With CPU affinity: time qemu-system-x86_64 \ -object thread-context,id=tc1,node-affinity=0 \ -object memory-backend-memfd,id=md1,hugetlb=on,hugetlbsize=2M,size=64G,prealloc-threads=12,prealloc=on,host-nodes=0,policy=bind,prealloc-context=tc1 \ -sandbox enable=on,resourcecontrol=deny \ -nographic -monitor stdio real 0m1.959s real 0m1.942s real 0m1.943s real 0m1.941s real 0m1.948s real 0m1.964s real 0m1.949s real 0m1.948s real 0m1.941s real 0m1.937s On reasonably large VMs, the speedup can be quite significant. While this concept is currently only used for short-lived preallocation threads, nothing major speaks against reusing the concept for other threads that are harder to identify/configure -- except that we need additional (idle) context threads that are otherwise left unused. This series does not yet tackle concurrent preallocation of memory backends. Memory backend objects are created and memory is preallocated one memory backend at a time -- and there is currently no way to do preallocation asynchronously. [1] https://lkml.kernel.org/r/ffdcd118d59b379ede2b64745144165a40f6a813.1652165704.git.mpriv...@redhat.com
I'm happy for more feedback and will wait some more. But if there is none, I'll most probably merge this myself by the end of next week.
Cheers! -- Thanks, David / dhildenb
