[Tmem-devel] Implementing Transcendent Memory on Lguest

[Nitin Gupta]

Hi Dan,

On 02/08/2010 11:52 PM, Dan Magenheimer wrote:
> 
> Nice!  I have been out of the office most of the last month so
> am still getting unburied. :-(
> 
> Again admitting ignorance of virtio, I have two concerns, performance
> and coherence, both assuming that there is some temporary storage
> of the tmem data (e.g. in a "ring buffer" or equivalent).  Is
> there any such temporary storage, or does the data always get
> transferred directly to the hypervisor?
>

This is a simple prototype for "swap to hypervisor" kind of thing.
Complete description of this driver is present in the mail I pointed out.
For now, it sends every page that is swapped to /dev/vswap (visible inside
guest) to host.
 
> For example, for performance, a ring buffer would be an extra page
> copy; for coherence, data stored in a ring buffer might not get
> flushed by a tmem flush command.

I do not have any performance data, so cannot comment on that.
However, I could not understand your coherence issue: this vswap
driver simply sends swap page to hypervisor synchronously.

Nitin


> 
>> -----Original Message-----
>> From: Nitin Gupta [mailto:ngupta at vflare.org]
>> Sent: Monday, February 08, 2010 11:04 AM
>> To: Dan Magenheimer
>> Cc: Gaurav Kukreja; tmem-devel at oss.oracle.com; rusty at rustcorp.com.au
>> Subject: Re: [Tmem-devel] Implementing Transcendent Memory on Lguest
>>
>> On 02/08/2010 11:04 PM, Dan Magenheimer wrote:
>>>
>>> I'm not very familiar with virtio but tmem requires synchronous
>>> data transfer between the "tmem server" (hypervisor) and the
>>> "tmem client" (guest).  I suspect that virtio assumes an
>>> asynchronous model.  If a virtio device can be configured
>>> as a synchronous device then your idea might work.
>>>
>>
>> virtio can be made to work synchronously. For virtio version of
>> "preswap", refer this:
>> http://oss.oracle.com/pipermail/tmem-devel/2010-January.txt
>>
>> It creates "vswap" device which swaps directly over hypervisor.
>> Pages are transferred to host using virtio in synchronous manner.
>>
>> (Links to code are at the end of above archived mail)
>>
>> Nitin
>>
>>
>>
>>>
>>> =============
>>>
>>> From: Gaurav Kukreja [mailto:mailme.gaurav at gmail.com]
>>> Sent: Sunday, February 07, 2010 11:11 PM
>>> To: tmem-devel at oss.oracle.com; rusty at rustcorp.com.au
>>> Subject: [Tmem-devel] Implementing Transcendent Memory on Lguest
>>>
>>> Hi,
>>>
>>> I have been working on trying to implement Transcendent Memory in
>> lguest.
>>>
>>> So far, Tmem on lguest is just working very rudimentarily. I am
>> currently working, on virtio balloon driver for the lguest, trying to
>> get it to work.
>>>
>>> I was wondering, and maybe this is vague, whether is it possible to
>> create a implementation of Transcendent Memory, as a Virtio Device. I
>> have read Rusty's paper on Virtio. I am yet to find out of the
>> technical issues regarding the same.
>>>
>>> Let us please evaluate, whether, there would be any benefits of
>> implementing, Transcendent Memory as a Virtio device. I dont know,
>> anything about KVM, but would it then be possible to port Tmem, to KVM
>> easily?
>>>
>>> Regards,
>>>
>>
>