Re: [RFC] 0/4 Migration Cache Overview

Lee Schermerhorn
2006-02-21T18:49:51Z

On Tue, 2006-02-21 at 10:04 -0800, Christoph Lameter wrote:
> On Tue, 21 Feb 2006, Marcelo Tosatti wrote:
>
> > The idea was to create a "partition" inside the swapcache which allows
> > for mapping+offset->pfn translation _without_ actually occupying space
> > in the swap map (an idr table is used instead).
> >
> > But apparently Christoph's mechanism adds the PFN number into
> > the page table entry itself, thus fulfilling the requirement for
> > "mapping+offset"->pfn indexing required for removal of pages underneath
> > a living process. Is that right?
>
> Right. Swap ptes contain the index into swap space which I am using to
> preserve the information contained in the pte's of anonymous pages. Thus
> the existing swap ptes couild be used. There were just a few minor
> modifications to the swap functions required.
>

Are the swap ptes used for migrating pages still reserving swap space on
real swap devices? I thought this was what the migration cache was
trying to avoid. Now each running instance of direct migration limits
itself to MIGRATE_CHUNK_SIZE [currently] 256 pages, so if the system has
much swap space at all, this shouldn't place too much of a load on swap
space. But, it does require that one have SOME swap space to migrate,
right?

Lee