RE: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

[Dan Magenheimer]

> It's bad, but it's better than ooming.
> 
> The same thing happens with vcpus: you run 10 guests on one core, if
> they all wake up, your cpu is suddenly 10x slower and has 30000x
> interrupt latency (30ms vs 1us, assuming 3ms timeslices).  Your disks
> become slower as well.
> 
> It's worse with memory, so you try to swap as a last resort.  However,
> swap is still faster than a crashed guest.

Your analogy only holds when the host administrator is either
extremely greedy or stupid.  My analogy only requires some
statistical bad luck: Multiple guests with peaks and valleys
of memory requirements happen to have their peaks align.

> > Third, host swapping makes live migration much more difficult.
> > Either the host swap disk must be accessible to all machines
> > or data sitting on a local disk MUST be migrated along with
> > RAM (which is not impossible but complicates live migration
> > substantially).
> 
> kvm does live migration with swapping, and has no special code to
> integrate them.
>  :
> Don't know about vmware, but kvm supports page sharing, swapping, and
> live migration simultaneously.

Hmmm... I'll bet I can break it pretty easily.  I think the
case you raised that you thought would cause host OOM'ing
will cause kvm live migration to fail.

Or maybe not... when a guest is in the middle of a live migration,
I believe (in Xen), the entire guest memory allocation (possibly
excluding ballooned-out pages) must be simultaneously in RAM briefly
in BOTH the host and target machine.  That is, live migration is
not "pipelined".  Is this also true of KVM?  If so, your
statement above is just waiting a corner case to break it.
And if not, I expect you've got fault tolerance issues.

> > If you talk to VMware customers (especially web-hosting services)
> > that have attempted to use overcommit technologies that require
> > host-swapping, you will find that they quickly become allergic
> > to memory overcommit and turn it off.  The end users (users of
> > the VMs that inexplicably grind to a halt) complain loudly.
> > As a result, RAM has become a bottleneck in many many systems,
> > which ultimately reduces the utility of servers and the value
> > of virtualization.
> 
> Choosing the correct overcommit ratio is certainly not an easy task.
> However, just hoping that memory will be available when you need it is
> not a good solution.

Choosing the _optimal_ overcommit ratio is impossible without a
prescient knowledge of the workload in each guest.  Hoping memory
will be available is certainly not a good solution, but if memory
is not available guest swapping is much better than host swapping.
And making RAM usage as dynamic as possible and live migration
as easy as possible are keys to maximizing the benefits (and
limiting the problems) of virtualization.

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