RE: [RFC PATCH v3 1/2] mempinfd: Add new syscall to provide memory pin

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Jason Gunthorpe [mailto:jgg@xxxxxxxx]
> Sent: Thursday, February 11, 2021 7:04 AM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>; Wangzhou (B)
> <wangzhou1@xxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
> iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx;
> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-api@xxxxxxxxxxxxxxx; Andrew
> Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>;
> gregkh@xxxxxxxxxxxxxxxxxxx; kevin.tian@xxxxxxxxx; jean-philippe@xxxxxxxxxx;
> eric.auger@xxxxxxxxxx; Liguozhu (Kenneth) <liguozhu@xxxxxxxxxxxxx>;
> zhangfei.gao@xxxxxxxxxx; chensihang (A) <chensihang1@xxxxxxxxxxxxx>
> Subject: Re: [RFC PATCH v3 1/2] mempinfd: Add new syscall to provide memory
> pin
> 
> On Tue, Feb 09, 2021 at 10:22:47PM +0000, Song Bao Hua (Barry Song) wrote:
> 
> > The problem is that SVA declares we can use any memory of a process
> > to do I/O. And in real scenarios, we are unable to customize most
> > applications to make them use the pool. So we are looking for some
> > extension generically for applications such as Nginx, Ceph.
> 
> But those applications will suffer jitter even if their are using CPU
> to do the same work. I fail to see why adding an accelerator suddenly
> means the application owner will care about jitter introduced by
> migration/etc.

The only point for this is that when migration occurs on the accelerator,
the impact/jitter is much bigger than it does on CPU. Then the accelerator
might be unhelpful.

> 
> Again in proper SVA it should be quite unlikely to take a fault caused
> by something like migration, on the same likelyhood as the CPU. If
> things are faulting so much this is a problem then I think it is a
> system level problem with doing too much page motion.

My point is that single one SVA application shouldn't require system
to make global changes, such as disabling numa balancing, disabling
THP, to decrease page fault frequency by affecting other applications.

Anyway, guys are in lunar new year. Hopefully, we are getting more
real benchmark data afterwards to make the discussion more targeted.

> 
> Jason

Thanks
Barry