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Re: [Qemu-devel] RFC migration of zero pages
From: |
Orit Wasserman |
Subject: |
Re: [Qemu-devel] RFC migration of zero pages |
Date: |
Thu, 31 Jan 2013 12:31:13 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130110 Thunderbird/17.0.2 |
On 01/31/2013 11:48 AM, Gleb Natapov wrote:
> On Thu, Jan 31, 2013 at 09:47:24AM +0200, Orit Wasserman wrote:
>> On 01/31/2013 08:57 AM, Peter Lieven wrote:
>>> Hi,
>>>
>>> I just came across an idea and would like to have feedback if it makes
>>> sence or not.
>>>
>>> If a VM is started without preallocated memory all memory that has not been
>>> written to
>>> reads as zeros, right?
>> Hi,
>> No the memory will be unmapped (we allocate on demand).
>>> If a VM with a lot of unwritten memory is migrated or if the memory
>>> contains a lot
>>> of zeroed out memory (e.g. Windows or Linux guest with page sanitization)
>>> all this memory
>>> is allocated on the target during live migration. Especially with KSM this
>>> leads
>>> to the problem that this memory is allocated and might be not available
>>> completely as
>>> merging of the pages will happen async.
>>>
>>> Wouldn't it make sense to not send zero pages in the first round where the
>>> complete
>>> ram is sent (if it is detectable that we are in this stage)?
>> We send one byte per zero page at the moment (see is_dup_page) we can
>> further optimizing it
>> by not sending it.
>> I have to point out that this is a very idle guest and we need to work on a
>> loaded guest
>> which is the more hard problem in migration.
>>
>> Also I notice that the bottle neck in migrating unmapped pages is the
>> detection of those pages
>> because we map the pages in order to check them, for a large guest this is
>> very expensive as mapping a page
>> results in a page fault in the host.
>> So what will be very helpful is actually locating those pages without
>> mapping them
>> which looks very complicated.
>>
> What is wrong with mincore()?
Avi/Michael do you remember why mincore can't be used to check if a guest page
is unmapped?
>
> --
> Gleb.
>
Re: [Qemu-devel] RFC migration of zero pages, Gleb Natapov, 2013/01/31
Re: [Qemu-devel] RFC migration of zero pages, Gleb Natapov, 2013/01/31
Re: [Qemu-devel] RFC migration of zero pages, Avi Kivity, 2013/01/31
Re: [Qemu-devel] RFC migration of zero pages, Gleb Natapov, 2013/01/31
Re: [Qemu-devel] RFC migration of zero pages, Avi Kivity, 2013/01/31
Re: [Qemu-devel] RFC migration of zero pages, Michael S. Tsirkin, 2013/01/31
Re: [Qemu-devel] RFC migration of zero pages, Michael R. hines, 2013/01/31