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

[Liu, Yuan1]

> -----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

> --
> MST