RE: [RFC 0/2] kvm: Transcendent Memory (tmem) on KVM

[Dan Magenheimer]

> From: Konrad Rzeszutek Wilk
> Subject: Re: [RFC 0/2] kvm: Transcendent Memory (tmem) on KVM
> 
> On Thu, Mar 15, 2012 at 08:01:52PM +0200, Avi Kivity wrote:
> > On 03/15/2012 07:49 PM, Dan Magenheimer wrote:
> > >
> > > The "WasActive" patch (https://lkml.org/lkml/2012/1/25/300)
> > > is intended to avoid the streaming situation you are creating here.
> > > It increases the "quality" of cached pages placed into zcache
> > > and should probably also be used on the guest-side stubs (and/or maybe
> > > the host-side zcache... I don't know KVM well enough to determine
> > > if that would work).
> > >
> > > As Dave Hansen pointed out, the WasActive patch is not yet correct
> > > and, as akpm points out, pageflag bits are scarce on 32-bit systems,
> > > so it remains to be seen if the WasActive patch can be upstreamed.
> > > Or maybe there is a different way to achieve the same goal.
> > > But I wanted to let you know that the streaming issue is understood
> > > and needs to be resolved for some cleancache backends just as it was
> > > resolved in the core mm code.
> >
> > Nice.  This takes care of the tail-end of the streaming (the more
> > important one - since it always involves a cold copy).  What about the
> > other side?  Won't the read code invoke cleancache_get_page() for every
> > page? (this one is just a null hypercall, so it's cheaper, but still
> > expensive).
> 
> That is something we should fix - I think it was mentioned in the frontswap
> email thread the need for batching and it certainly seems required as those
> hypercalls aren't that cheap.

And exactly how expensive ARE hypercalls these days?  On the first VT/SVN
systems they were tens of thousands of cycles... now they are closer
to sub-thousand are they not?  (I remember seeing a graph of hypercall
overhead dropping across generations of CPUs... anybody have a pointer to
a public graph of this?)

One of my favorite papers these days is "When Poll is Better than Interrupt"
(http://static.usenix.org/events/fast12/tech/full_papers/Yang.pdf) which
argues that wasting some CPU cycles doing a busy-wait is often more
efficient than slogging through the Block I/O subsystem to set up
and respond to an interrupt, if the device is fast enough.  I wonder if the
same might be true comparing hypercall overhead for tmem vs the path for
KVM to get a page from the host via its normal path?

Ignoring that for now, if excessive hypercalls is a problem, a better
solution than batching may be to modify the Maharashtra approach to
be more like RAMster:  Put zcache in the guest-side and treat the
host like a "remote" system.

But let's wait for the Maharashta team to do some measurements first
before we make any assumptions or change any designs...
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