On Mon, Jun 19, 2017 at 12:26:52PM +0200, David Hildenbrand wrote:
> On 19.06.2017 12:08, Stefan Hajnoczi wrote:
> > On Fri, Jun 16, 2017 at 04:20:02PM +0200, David Hildenbrand wrote:
> >> Important restrictions of this concept:
> >> - Guests without a virtio-mem guest driver can't see that memory.
> >> - We will always require some boot memory that cannot get unplugged.
> >> Also, virtio-mem memory (as all other hotplugged memory) cannot become
> >> DMA memory under Linux. So the boot memory also defines the amount of
> >> DMA memory.
> >
> > I didn't know that hotplug memory cannot become DMA memory.
> >
> > Ouch. Zero-copy disk I/O with O_DIRECT and network I/O with virtio-net
> > won't be possible.
> >
> > When running an application that uses O_DIRECT file I/O this probably
> > means we now have 2 copies of pages in memory: 1. in the application and
> > 2. in the kernel page cache.
> >
> > So this increases pressure on the page cache and reduces performance :(.
> >
> > Stefan
> >
>
> arch/x86/mm/init_64.c:
>
> /*
> * Memory is added always to NORMAL zone. This means you will never get
> * additional DMA/DMA32 memory.
> */
> int arch_add_memory(int nid, u64 start, u64 size, bool for_device)
> {
>
> The is for sure something to work on in the future. Until then, base
> memory of 3.X GB should be sufficient, right?
I'm not sure that helps because applications typically don't control
where their buffers are located?
Stefan
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