Re: [PATCH v2 0/7] hostmem: NUMA-aware memory preallocation using ThreadContext

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* David Hildenbrand (david@redhat.com) wrote:
> On 10.10.22 12:40, Dr. David Alan Gilbert wrote:
> > * David Hildenbrand (david@redhat.com) 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.
> 
> Hi Dave,
> 
> > 
> > Since you seem to have a full set of r-b's - do you intend to merge this
> > as-is or do the cuncurrenct preallocation first?
> 
> I intent to merge this as is, as it provides a benefit as it stands and
> concurrent preallcoation might not require user interface changes.

Yep, that's fair enough.

> I do have some ideas on how to implement concurrent preallocation, but it
> needs more thought (and more importantly, time).

Yep, it would be nice for the really huge VMs.

Dave


> -- 
> Thanks,
> 
> David / dhildenb
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK