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 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?

> > --
> > MST