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

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Date:	Fri, 30 Apr 2010 09:43:55 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>, Pavel Machek <pavel@....cz>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, jeremy@...p.org,
	hugh.dickins@...cali.co.uk, ngupta@...are.org, JBeulich@...ell.com,
	chris.mason@...cle.com, kurt.hackel@...cle.com,
	dave.mccracken@...cle.com, npiggin@...e.de,
	akpm@...ux-foundation.org, riel@...hat.com
Subject: RE: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

(I'll back down on the CMM2 comparisons until I can go
back and read the paper :-)

> >> [frontswap is] really
> >> not very different from a synchronous swap device.
> >>
> > Not to beat a dead horse, but there is a very key difference:
> > The size and availability of frontswap is entirely dynamic;
> > any page-to-be-swapped can be rejected at any time even if
> > a page was previously successfully swapped to the same index.
> > Every other swap device is much more static so the swap code
> > assumes a static device.  Existing swap code can account for
> > "bad blocks" on a static device, but this is far from sufficient
> > to handle the dynamicity needed by frontswap.
> 
> Given that whenever frontswap fails you need to swap anyway, it is
> better for the host to never fail a frontswap request and instead back
> it with disk storage if needed.  This way you avoid a pointless vmexit
> when you're out of memory.  Since it's disk backed it needs to be
> asynchronous and batched.
> 
> At this point we're back with the ordinary swap API.  Simply have your
> host expose a device which is write cached by host memory, you'll have
> all the benefits of frontswap with none of the disadvantages, and with
> no changes to guest .

I think you are making a number of possibly false assumptions here:
1) The host [the frontswap backend may not even be a hypervisor]
2) can back it with disk storage [not if it is a bare-metal hypervisor]
3) avoid a pointless vmexit [no vmexit for a non-VMX (e.g. PV) guest]
4) when you're out of memory [how can this be determined outside of
   the hypervisor?]

And, importantly, "have your host expose a device which is write
cached by host memory"... you are implying that all guest swapping
should be done to a device managed/controlled by the host?  That
eliminates guest swapping to directIO/SRIOV devices doesn't it?

Anyway, I think we can see now why frontswap might not be a good
match for a hosted hypervisor (KVM), but that doesn't make it
any less useful for a bare-metal hypervisor (or TBD for in-kernel
compressed swap and TBD for possible future pseudo-RAM technologies).

Dan
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