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

-----Original Message-----
From: Jason Gunthorpe [mailto:jgg@ziepe.ca]
Sent: Thursday, February 11, 2021 7:04 AM
To: Song Bao Hua (Barry Song) <redacted>
Cc: David Hildenbrand <redacted>; Wangzhou (B)
[off-list ref]; linux-kernel@vger.kernel.org;
iommu@lists.linux-foundation.org; linux-mm@kvack.org;
linux-arm-kernel@lists.infradead.org; linux-api@vger.kernel.org; Andrew
Morton [off-list ref]; Alexander Viro [off-list ref];
gregkh@linuxfoundation.org; kevin.tian@intel.com; jean-philippe@linaro.org;
eric.auger@redhat.com; Liguozhu (Kenneth) [off-list ref];
zhangfei.gao@linaro.org; chensihang (A) [off-list ref]
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:

quoted

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

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