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

["Michael S. Tsirkin" <mst@redhat.com>]

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?

> This is likely to happen in the first round of migration
> memory iteration.

Try testing this and see then?


-- 
MST