On 19.06.2017 12:08, Stefan Hajnoczi wrote:

On Fri, Jun 16, 2017 at 04:20:02PM +0200, David Hildenbrand wrote:

quoted

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?

-- 

Thanks,

David

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>