Re: [PATCH v5 00/13] WIP: Use Intel DSA accelerator to offload zero page checking in multifd live migration.

[Michael S. Tsirkin]

On Mon, Jul 15, 2024 at 03:23:13PM +0000, Liu, Yuan1 wrote:
> > -----Original Message-----
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Monday, July 15, 2024 10:43 PM
> > To: Liu, Yuan1 <yuan1.liu@intel.com>
> > Cc: Wang, Yichen <yichen.wang@bytedance.com>; Paolo Bonzini
> > <pbonzini@redhat.com>; Marc-André Lureau <marcandre.lureau@redhat.com>;
> > Daniel P. Berrangé <berrange@redhat.com>; Thomas Huth <thuth@redhat.com>;
> > Philippe Mathieu-Daudé <philmd@linaro.org>; Peter Xu <peterx@redhat.com>;
> > Fabiano Rosas <farosas@suse.de>; Eric Blake <eblake@redhat.com>; Markus
> > Armbruster <armbru@redhat.com>; Cornelia Huck <cohuck@redhat.com>; qemu-
> > devel@nongnu.org; Hao Xiang <hao.xiang@linux.dev>; Kumar, Shivam
> > <shivam.kumar1@nutanix.com>; Ho-Ren (Jack) Chuang
> > <horenchuang@bytedance.com>
> > Subject: Re: [PATCH v5 00/13] WIP: Use Intel DSA accelerator to offload
> > zero page checking in multifd live migration.
> > 
> > On Mon, Jul 15, 2024 at 01:09:59PM +0000, Liu, Yuan1 wrote:
> > > > -----Original Message-----
> > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > Sent: Monday, July 15, 2024 8:24 PM
> > > > To: Liu, Yuan1 <yuan1.liu@intel.com>
> > > > Cc: Wang, Yichen <yichen.wang@bytedance.com>; Paolo Bonzini
> > > > <pbonzini@redhat.com>; Marc-André Lureau
> > <marcandre.lureau@redhat.com>;
> > > > Daniel P. Berrangé <berrange@redhat.com>; Thomas Huth
> > <thuth@redhat.com>;
> > > > Philippe Mathieu-Daudé <philmd@linaro.org>; Peter Xu
> > <peterx@redhat.com>;
> > > > Fabiano Rosas <farosas@suse.de>; Eric Blake <eblake@redhat.com>;
> > Markus
> > > > Armbruster <armbru@redhat.com>; Cornelia Huck <cohuck@redhat.com>;
> > qemu-
> > > > devel@nongnu.org; Hao Xiang <hao.xiang@linux.dev>; Kumar, Shivam
> > > > <shivam.kumar1@nutanix.com>; Ho-Ren (Jack) Chuang
> > > > <horenchuang@bytedance.com>
> > > > Subject: Re: [PATCH v5 00/13] WIP: Use Intel DSA accelerator to
> > offload
> > > > zero page checking in multifd live migration.
> > > >
> > > > On Mon, Jul 15, 2024 at 08:29:03AM +0000, Liu, Yuan1 wrote:
> > > > > > -----Original Message-----
> > > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > > Sent: Friday, July 12, 2024 6:49 AM
> > > > > > To: Wang, Yichen <yichen.wang@bytedance.com>
> > > > > > Cc: Paolo Bonzini <pbonzini@redhat.com>; Marc-André Lureau
> > > > > > <marcandre.lureau@redhat.com>; Daniel P. Berrangé
> > > > <berrange@redhat.com>;
> > > > > > Thomas Huth <thuth@redhat.com>; Philippe Mathieu-Daudé
> > > > > > <philmd@linaro.org>; Peter Xu <peterx@redhat.com>; Fabiano Rosas
> > > > > > <farosas@suse.de>; Eric Blake <eblake@redhat.com>; Markus
> > Armbruster
> > > > > > <armbru@redhat.com>; Cornelia Huck <cohuck@redhat.com>; qemu-
> > > > > > devel@nongnu.org; Hao Xiang <hao.xiang@linux.dev>; Liu, Yuan1
> > > > > > <yuan1.liu@intel.com>; Kumar, Shivam <shivam.kumar1@nutanix.com>;
> > Ho-
> > > > Ren
> > > > > > (Jack) Chuang <horenchuang@bytedance.com>
> > > > > > Subject: Re: [PATCH v5 00/13] WIP: Use Intel DSA accelerator to
> > > > offload
> > > > > > zero page checking in multifd live migration.
> > > > > >
> > > > > > On Thu, Jul 11, 2024 at 02:52:35PM -0700, Yichen Wang wrote:
> > > > > > > * Performance:
> > > > > > >
> > > > > > > We use two Intel 4th generation Xeon servers for testing.
> > > > > > >
> > > > > > > Architecture:        x86_64
> > > > > > > CPU(s):              192
> > > > > > > Thread(s) per core:  2
> > > > > > > Core(s) per socket:  48
> > > > > > > Socket(s):           2
> > > > > > > NUMA node(s):        2
> > > > > > > Vendor ID:           GenuineIntel
> > > > > > > CPU family:          6
> > > > > > > Model:               143
> > > > > > > Model name:          Intel(R) Xeon(R) Platinum 8457C
> > > > > > > Stepping:            8
> > > > > > > CPU MHz:             2538.624
> > > > > > > CPU max MHz:         3800.0000
> > > > > > > CPU min MHz:         800.0000
> > > > > > >
> > > > > > > We perform multifd live migration with below setup:
> > > > > > > 1. VM has 100GB memory.
> > > > > > > 2. Use the new migration option multifd-set-normal-page-ratio to
> > > > control
> > > > > > the total
> > > > > > > size of the payload sent over the network.
> > > > > > > 3. Use 8 multifd channels.
> > > > > > > 4. Use tcp for live migration.
> > > > > > > 4. Use CPU to perform zero page checking as the baseline.
> > > > > > > 5. Use one DSA device to offload zero page checking to compare
> > with
> > > > the
> > > > > > baseline.
> > > > > > > 6. Use "perf sched record" and "perf sched timehist" to analyze
> > CPU
> > > > > > usage.
> > > > > > >
> > > > > > > A) Scenario 1: 50% (50GB) normal pages on an 100GB vm.
> > > > > > >
> > > > > > >   CPU usage
> > > > > > >
> > > > > > >   |---------------|---------------|---------------|-------------
> > > > --|
> > > > > > >   |               |comm           |runtime(msec)  
> > > > > > > |totaltime(msec)|
> > > > > > >   |---------------|---------------|---------------|-------------
> > > > --|
> > > > > > >   |Baseline       |live_migration |5657.58        |               
> > > > > > > |
> > > > > > >   |               |multifdsend_0  |3931.563       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_1  |4405.273       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_2  |3941.968       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_3  |5032.975       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_4  |4533.865       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_5  |4530.461       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_6  |5171.916       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_7  |4722.769       |41922          
> > > > > > > |
> > > > > > >   |---------------|---------------|---------------|-------------
> > > > --|
> > > > > > >   |DSA            |live_migration |6129.168       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_0  |2954.717       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_1  |2766.359       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_2  |2853.519       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_3  |2740.717       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_4  |2824.169       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_5  |2966.908       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_6  |2611.137       |               
> > > > > > > |
> > > > > > >   |               |multifdsend_7  |3114.732       |               
> > > > > > > |
> > > > > > >   |               |dsa_completion |3612.564       |32568          
> > > > > > > |
> > > > > > >   |---------------|---------------|---------------|-------------
> > > > --|
> > > > > > >
> > > > > > > Baseline total runtime is calculated by adding up all
> > multifdsend_X
> > > > > > > and live_migration threads runtime. DSA offloading total runtime
> > is
> > > > > > > calculated by adding up all multifdsend_X, live_migration and
> > > > > > > dsa_completion threads runtime. 41922 msec VS 32568 msec runtime
> > and
> > > > > > > that is 23% total CPU usage savings.
> > > > > >
> > > > > >
> > > > > > Here the DSA was mostly idle.
> > > > > >
> > > > > > Sounds good but a question: what if several qemu instances are
> > > > > > migrated in parallel?
> > > > > >
> > > > > > Some accelerators tend to basically stall if several tasks
> > > > > > are trying to use them at the same time.
> > > > > >
> > > > > > Where is the boundary here?
> > > > >
> > > > > A DSA device can be assigned to multiple Qemu instances.
> > > > > The DSA resource used by each process is called a work queue, each
> > DSA
> > > > > device can support up to 8 work queues and work queues are
> > classified
> > > > into
> > > > > dedicated queues and shared queues.
> > > > >
> > > > > A dedicated queue can only serve one process. Theoretically, there
> > is no
> > > > limit
> > > > > on the number of processes in a shared queue, it is based on enqcmd
> > +
> > > > SVM technology.
> > > > >
> > > > > https://www.kernel.org/doc/html/v5.17/x86/sva.html
> > > >
> > > > This server has 200 CPUs which can thinkably migrate around 100 single
> > > > cpu qemu instances with no issue. What happens if you do this with
> > DSA?
> > >
> > > First, the DSA work queue needs to be configured in shared mode, and one
> > > queue is enough.
> > >
> > > The maximum depth of the work queue of the DSA hardware is 128, which
> > means
> > > that the number of zero-page detection tasks submitted cannot exceed
> > 128,
> > > otherwise, enqcmd will return an error until the work queue is available
> > again
> > >
> > > 100 Qemu instances need to be migrated concurrently, I don't have any
> > data on
> > > this yet, I think the 100 zero-page detection tasks can be successfully
> > submitted
> > > to the DSA hardware work queue, but the throughput of DSA's zero-page
> > detection also
> > > needs to be considered. Once the DSA maximum throughput is reached, the
> > work queue
> > > may be filled up quickly, this will cause some Qemu instances to be
> > temporarily unable
> > > to submit new tasks to DSA.
> > 
> > The unfortunate reality here would be that there's likely no QoS, this
> > is purely fifo, right?
> 
> Yes, this scenario may be fifo, assuming that the number of pages each task
> is the same, because DSA hardware consists of multiple work engines, they can
> process tasks concurrently, usually in a round-robin way to get tasks from the
> work queue.   
> 
> DSA supports priority and flow control based on work queue granularity.
> https://github.com/intel/idxd-config/blob/stable/Documentation/accfg/accel-config-config-wq.txt

Right but it seems clear there aren't enough work queues for a typical setup.

> > > This is likely to happen in the first round of migration
> > > memory iteration.
> > 
> > Try testing this and see then?
> 
> Yes, I can test based on this patch set. Please review the test scenario
> My server has 192 CPUs, and 8 DSA devices, 100Gbps NIC.
> All 8 DSA devices serve 100 Qemu instances for simultaneous live migration.
> Each VM has 1 vCPU, and 1G memory, with no workload in the VM.
> 
> You want to know if some Qemu instances are stalled because of DSA, right?

And generally just run same benchmark you did compared to cpu:
worst case and average numbers would be interesting.

> > --
> > MST