From: Stefan Hajnoczi <stefanha@gmail.com>
To: David Hildenbrand <david@redhat.com>
Cc: KVM <kvm@vger.kernel.org>,
"virtualization@lists.linux-foundation.org"
<virtualization@lists.linux-foundation.org>,
"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
Andrea Arcangeli <aarcange@redhat.com>,
"Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [RFC] virtio-mem: paravirtualized memory
Date: Wed, 21 Jun 2017 12:08:17 +0100 [thread overview]
Message-ID: <20170621110817.GF16183@stefanha-x1.localdomain> (raw)
In-Reply-To: <4cec825b-d92e-832e-3a76-103767032528@redhat.com>
[-- Attachment #1: Type: text/plain, Size: 1509 bytes --]
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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next prev parent reply other threads:[~2017-06-21 11:08 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-16 14:20 [RFC] virtio-mem: paravirtualized memory David Hildenbrand
2017-06-16 15:04 ` Michael S. Tsirkin
2017-06-16 15:59 ` David Hildenbrand
2017-06-16 20:19 ` Michael S. Tsirkin
2017-06-18 10:17 ` David Hildenbrand
2017-06-19 10:08 ` Stefan Hajnoczi
2017-06-19 10:26 ` David Hildenbrand
2017-06-21 11:08 ` Stefan Hajnoczi [this message]
2017-06-21 12:32 ` David Hildenbrand
2017-06-23 12:45 ` Stefan Hajnoczi
2017-07-25 8:21 ` David Hildenbrand
2017-07-28 11:09 ` David Hildenbrand
2017-07-28 15:16 ` Dan Williams
2017-07-28 15:48 ` David Hildenbrand
2017-07-31 14:12 ` Michael S. Tsirkin
2017-07-31 15:04 ` David Hildenbrand
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