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.
