Subject: Re: [PATCH v3 net-next 08/14] mlx4: use order-0 pages for RX

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 14 Feb 2017 20:38:22 +0100

On Tue, 14 Feb 2017 12:04:26 -0500 (EST)
David Miller <[email-masked]> wrote:

> From: Tariq Toukan <[email-masked]>
> Date: Tue, 14 Feb 2017 16:56:49 +0200
> 
> > Internally, I already implemented "dynamic page-cache" and
> > "page-reuse" mechanisms in the driver, and together they totally
> > bridge the performance gap.  

It sounds like you basically implemented a page_pool scheme...

> I worry about a dynamically growing page cache inside of drivers
> because it is invisible to the rest of the kernel.

Exactly, that is why I wanted a separate standardized thing, I call the
page_pool, which is part of the MM-tree and interacts with the page
allocator.  E.g. it must implement/support a way the page allocator can
reclaim pages from it (admit I didn't implement this in RFC patches).

> It responds only to local needs.

Generally true, but a side effect of recycling these pages, result in
less fragmentation of the page allocator/buddy system.

> The price of the real page allocator comes partly because it can
> respond to global needs.
> 
> If a driver consumes some unreasonable percentage of system memory, it
> is keeping that memory from being used from other parts of the system
> even if it would be better for networking to be slightly slower with
> less cache because that other thing that needs memory is more
> important.

(That is why I want to have OOM protection at device level, with the
recycle feedback from page pool we have this knowledge, and further I
wanted to allow blocking a specific RX queue[1])
[1] https://prototype-kernel.readthedocs.io/en/latest/vm/page_pool/design/memory_model_nic.html#userspace-delivery-and-oom

> I think this is one of the primary reasons that the MM guys severely
> chastise us when we build special purpose local caches into networking
> facilities.
> 
> And the more I think about it the more I think they are right.

+1
 
> One path I see around all of this is full integration.  Meaning that
> we can free pages into the page allocator which are still DMA mapped.
> And future allocations from that device are prioritized to take still
> DMA mapped objects.

I like this idea.  Are you saying that this should be done per DMA
engine or per device?

If this is per device, it is almost the page_pool idea.  

> Yes, we still need to make the page allocator faster, but this kind of
> work helps everyone not just 100GB ethernet NICs.

True.  And Mel already have some generic improvements to the page
allocator queued for the next merge.  And I have the responsibility to
get the bulking API into shape.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

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