Re: madvise (MADV_FREE)

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Wed, Mar 22, 2000 at 04:39:12PM -0500, Chuck Lever wrote:
> 
> in fact, i'd say it is safe in general to lower DEFAULT_MMAP_THRESHOLD to
> the system page size.  that way you'd get closer to the behavior you're
> after, and you'd also win a much bigger effective heap size when
> allocating large objects, because you can only allocate up to 960M of a
> process's address space with sbrk().

You can use MADV_DONTNEED to reclaim demand-zero pages below sbrk()
even without using memory map in the first place, and I understand that
recent versions of glibc will resort to extending the heap with mmap()
automatically once sbrk() reaches its limit.  So, I don't think that
decreasing DEFAULT_MMAP_THRESHOLD really gains that much.
> 
> to say this another way, the page mapping binds a virtual address to a
> page in the page cache. MADV_DONTNEED simply removes that binding.  
> normal page aging will discover the unbound pages in the page cache and
> remove them.  so really, MADV_DONTNEED is actually disconnected from the
> mechanism of swapping or discarding the page's data.

Not for anonymous pages, where the pte reference is the _only_ reference
to the page (except for swap-cached pages).  In this case, MADV_DONTNEED
will genuinely free the page.

> nah, i still say a better way to handle this case is to lower malloc's
> "use an anon map instead of the heap" threshold to 4K or 8K.  right now
> it's 32K by default.  

No, it's much cheaper to do a MADV_DONTNEED when freeing an anonymous
page: that way the pageout and subsequent demand-zero pagein all happen
entirely within the page tables, without having to perform lots of
operations on the vma tree of the process.

--Stephen

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