[Tmem-devel] Implementing Transcendent Memory on Lguest

[Dan Magenheimer]

> However, I could not understand your coherence issue: this vswap
> driver simply sends swap page to hypervisor synchronously.

I am asking if anything in the virtio "layer" buffers
the data, making it appear to the client that the data
has been copied from/to the hypervisor (and all of tmem's
data structures updated in the hypervisor) but it really
hasn't happened yet. If so, bad things can happen.

A quick scan of Rusty's OSR paper shows VIRTIO_RING
which sounds like to me like there might be some buffering.
If I am wrong, that's OK, but I wanted to double-check.

Or even if the exiting implementation of virtio doesn't
do this but the the architecture/design of virtio
allows for this to happen, I'd be concerned.

Thanks,
Dan

> -----Original Message-----
> From: Nitin Gupta [mailto:ngupta at vflare.org]
> Sent: Monday, February 08, 2010 11:30 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
> 
> 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,
> >>>
> >>
> >
>