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[82.132.238.187]) by smtp.gmail.com with ESMTPSA id b1-20020a1709063ca100b007305d408b3dsm6565625ejh.78.2022.10.11.02.02.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 11 Oct 2022 02:02:48 -0700 (PDT) Date: Tue, 11 Oct 2022 10:02:45 +0100 From: "Dr. David Alan Gilbert" To: David Hildenbrand Cc: qemu-devel@nongnu.org, Michal Privoznik , Igor Mammedov , "Michael S. Tsirkin" , Paolo Bonzini , Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , Eduardo Habkost , Eric Blake , Markus Armbruster , Richard Henderson , Stefan Weil Subject: Re: [PATCH v2 0/7] hostmem: NUMA-aware memory preallocation using ThreadContext Message-ID: References: <23dd0ce0-5393-3aa0-affe-11277c6a123b@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <23dd0ce0-5393-3aa0-affe-11277c6a123b@redhat.com> User-Agent: Mutt/2.2.7 (2022-08-07) Received-SPF: pass client-ip=170.10.133.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * 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