Re: [RFC] madvise(MADV_TRUNCATE)

[Theodore Ts'o <tytso@mit.edu>]

On Fri, Oct 28, 2005 at 07:55:09PM +0200, Blaisorblade wrote:
> > On Thu, Oct 27, 2005 at 06:42:36PM -0700, Badari Pulavarty wrote:
> > > Like Andrea mentioned MADV_DONTNEED should be able to do what JVM
> > > folks want. If they want more than that, get in touch with me.
> > > While doing MADV_REMOVE, I will see if I can satsify their needs also.
> 
> > Well, I asked if what he wanted was simply walking all of the page
> > tables and marking the indicated pages as "clean",
> This idea sounds interesting and kludgy enough :-) .
> > but he claimed that 
> > anything that involved walking the pages tables would be too slow.
> > But it may be that he was assuming this would be as painful as
> > munmap(), when of course it wouldn't be.
> 
> I am curious which is the difference between the two. I know that we must also 
> walk the vma tree, and that since we bundle the pointers in the vma the 
> spatial locality is very poor, but I still don't get this huge loss.

Because if we do an munmap, we're removing entries from the page table
entries, which means we have to do cross-CPU IPI's to flush TLB's on
all of the CPU's.  That wouldn't be necessary if we're just marking
the pages clean.

						- Ted

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