Re: [Xen-devel] windows tmem

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: James Harper [mailto:james.harper@xxxxxxxxxxxxxxxx]
> Sent: 02 June 2013 08:37
> To: James Harper; Konrad Rzeszutek Wilk
> Cc: Paul Durrant; xen-devel@xxxxxxxxxxxxx
> Subject: RE: [Xen-devel] windows tmem
> 
> >
> > Unfortunately it gets worse... I'm testing on windows 2003 at the moment,
> > and it seems to always write out data in 64k chunks, which are aligned to a
> 4k
> > boundary. Then it reads in one or more of those pages, and maybe later re-
> > uses the same part of the swapfile for something else. It seems that all
> reads
> > are 4k in size, but there may be some grouping of those requests at a lower
> > layer.
> >
> > So I would end up caching up to 16x the actual data, with no way of
> knowing
> > which of those 16 pages are actually being swapped out and which are just
> > optimistically being written to disk without actually being paged out.
> >
> > I'll do a bit of analysis of the MDL being written as that may give me some
> > more information but it's not looking as good as I'd hoped.
> >
> 
> I now have a working implementation that does write-through caching of
> pagefile writes to ephemeral tmem. It keeps some counters on get and put
> operations, and on a Windows 2003 server with 256MB memory assigned,
> after a bit of running and flipping between applications I get:
> 
> put_success_count = 96565
> put_fail_count    = 0
> get_success_count = 34514
> get_fail_count    = 5369
> 
> which is somewhere around 85% hit rate vs misses. That seems pretty good
> except that windows is quite aggressive about paging out, so there are a lot
> of unused writes (and therefore tmem usage) and I'm not sure if it's a net
> win.
> 
> Subjectively, windows does seem faster with my driver active, but I'm using
> it over a crappy adsl connection so it's hard to measure in any precise way.
> 
> I'm trying to see if it is possible to use tmem as a page cache cache which
> would be more useful but I'm not yet sure if the required hooks exist in fs
> minifilters.
> 

Do you have any numbers for a more recent version of Windows? At least a 6.x 
kernel. Perhaps the pageout characteristics are better?

  Paul

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