Re: [Xen-devel] windows tmem

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, May 28, 2013 at 10:17:00AM -0400, Konrad Rzeszutek Wilk wrote:
> On Tue, May 28, 2013 at 09:53:23AM +0000, James Harper wrote:
> > > > A less risky implementation would just use tmem as a write-thru cache 
> > > > and
> > > > then just throw out old pages on an LRU basis or something. Or discard 
> > > > the
> > > > pages from tmem on read but write them back to disk. It kind of sucks 
> > > > the
> > > > usefulness out of it though if you can't avoid the writes, and if 
> > > > windows is
> > > > doing some trickery to page out during periods of low io then I'd be
> > > > upsetting that too.
> > > >
> > > 
> > > This sounds a lot less fragile and the saving on reads to the storage 
> > > backend
> > > could still be significant.
> > 
> > Should be easy enough to test I guess.
> > 
> > > > Anyway I have written a skeleton fs filter so I can monitor what is 
> > > > going on
> > > > in
> > > > better detail when I get a few minutes. Later versions of Windows might
> > > > make use of discard (trim/unmap) which would solve most of the above
> > > > problems.
> > > >
> > > > There do seem to be some (windows equivalent of) page cache operations
> > > > that could be hooked too... or else the api callback naming is leading 
> > > > me
> > > > astray.
> > > >
> > > 
> > > Sounds interesting. Presumably, if you can reliably intercept all IO on a
> > > pagefile then I guess you could use tmem as a write-back cache in front of
> > > doing your own file i/o down the storage stack, as long as you could 
> > > reliably
> > > flush it out when necessary. E.g. does windows assume anything about the
> > > pagefile content on resume from S3 or S4?
> > > 
> > 
> > I need to look up if FS filter is notified about power state transitions. 
> > There may be a FLUSH of some sort that happens at that time. Newer versions 
> > of windows have a thing called 'hybrid suspend', where the hibernate file 
> > is written out as if windows were about to be hibernated, but it goes to 
> > sleep instead of hibernating but if power is lost a resume is still 
> > possible. It may be acceptable to say that tmem = no hibernate. Migrate 
> > should be easy enough as I have direct control over that and can make tmem 
> > be written back out to the pagefile first.
> > 
> > This all assumes that write back is possible too...
> 
> I am not familiar with the Windows APIs, but it sounds like you
> want to use the tmem ephermeal disk cache as an secondary cache
> (which is BTW what Linux does too).
> 
> That is OK the only thing you need to keep in mind that the
> hypervisor might flush said cache out if it decides to do it
> (say a new guest is launched and it needs the memory that
> said cache is using).
> 
> So the tmem_get might tell that it does not have the page anymore.

Oh and I should mention that I would be more than thrilled to try
this out and see how it works.

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