RE: [GIT PULL] mm: frontswap (for 3.2 window)

[Dan Magenheimer]

> From: Avi Kivity [mailto:avi@xxxxxxxxxx]
> Subject: Re: [GIT PULL] mm: frontswap (for 3.2 window)
> 
> On 11/02/2011 06:02 PM, Andrea Arcangeli wrote:
> > Hi Avi,
> >
> > On Wed, Nov 02, 2011 at 05:44:50PM +0200, Avi Kivity wrote:
> > > If you look at cleancache, then it addresses this concern - it extends
> > > pagecache through host memory.  When dropping a page from the tail of
> > > the LRU it first goes into tmem, and when reading in a page from disk
> > > you first try to read it from tmem.  However in many workloads,
> > > cleancache is actually detrimental.  If you have a lot of cache misses,
> > > then every one of them causes a pointless vmexit; considering that
> > > servers today can chew hundreds of megabytes per second, this adds up.
> > > On the other side, if you have a use-once workload, then every page that
> > > falls of the tail of the LRU causes a vmexit and a pointless page copy.
> >
> > I also think it's bad design for Virt usage, but hey, without this
> > they can't even run with cache=writeback/writethrough and they're
> > forced to cache=off, and then they claim specvirt is marketing, so for
> > Xen it's better than nothing I guess.
> 
> Surely Xen can use the pagecache, it uses Linux for I/O just like kvm.
> 
> > I'm trying right now to evaluate it as a pure zcache host side
> > optimization.
> 
> zcache style usage is fine.  It's purely internal so no ABI constraints,
> and no hypercalls either.  It's still synchronous though so RAMster like
> approaches will not work well.

Still experimental, but only the initial local put must be synchronous.
RAMster uses a separate thread to "remotify" pre-compressed pages.
The "get" still needs to be synchronous, but (if I ever have time to
get back to coding it) I've got some ideas on how to fix that.  If
I manage to get that working, perhaps it could be used for Andrea's
write-precompressed-zcache-pages-to-disk.

Dan
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