linux-kernel - Re: [PATCH v2] nd_blk: add support for "read flush" DSM flag

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Date:	Thu, 20 Aug 2015 13:00:10 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Luis R. Rodriguez" <mcgrof@...e.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...e.de>, Christoph Hellwig <hch@....de>,
	Christoph Jaeger <cj@...ux.com>,
	Dan Streetman <ddstreet@...e.org>,
	Ingo Molnar <mingo@...hat.com>,
	Juergen Gross <jgross@...e.com>, Len Brown <lenb@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Thierry Reding <treding@...dia.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Toshi Kani <toshi.kani@...com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Will Deacon <will.deacon@....com>,
	Linux ACPI <linux-acpi@...r.kernel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	X86 ML <x86@...nel.org>
Subject: Re: [PATCH v2] nd_blk: add support for "read flush" DSM flag

On Thu, 2015-08-20 at 11:26 -0700, Dan Williams wrote:
> On Thu, Aug 20, 2015 at 11:17 AM, Ross Zwisler
> <ross.zwisler@...ux.intel.com> wrote:
> > On Thu, 2015-08-20 at 10:59 -0700, Dan Williams wrote:
> [..]
> > Ah, I think we're getting confused about the deinterleave part.
> >
> > The aperture is a set of contiguous addresses from the perspective of the
> > DIMM, but when it's interleaved by the iMC it becomes a bunch of segments that
> > are not contiguous in the virtual address space of the kernel.
> >
> > Meaning, say you have an 8k aperture that is interleaved with one other DIMM
> > on a 256 byte granularity - this means that in SPA space you'll end up with a
> > big mesh of 256 byte chunks, half of which belong to you and half which don't:
> >
> > SPA space:
> > +--------------------+
> > |256 bytes (ours)    |
> > +--------------------+
> > |256 bytes (not ours)|
> > +--------------------+
> > |256 bytes (ours)    |
> > +--------------------+
> > |256 bytes (not ours)|
> > +--------------------+
> > ...
> >
> > To be able to flush the entire aperture unconditionally, we have to walk
> > through all the segments that belong to use and flush each one of them.  I
> > don't think we want to blindly flush the entire interleaved space because a)
> > the other chunks are some other DIMMs' apertures, and b) we'd be flushing 2x
> > or more (depending on how many DIMMs are interleaved) the space we need, one
> > cache line at a time.
> 
> I am indeed proposing flushing other DIMMs because those ranges are
> invalidated by the aperture moving.  This is based on the assumption
> that the flushing is cheaper in the case when no dirty-lines are
> found.  The performance gains of doing piecemeal flushes seems not
> worth the complexity.

Why are the segments belonging to other apertures invalidated because we have
moved our aperture?  They are all independent cache lines (segments must be a
multiple of the cache line size), and the other apertures might be in the
middle of some other I/O operation on some other CPU that we know nothing
about.



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