Re: madvise (MADV_FREE)

[Jamie Lokier <jamie.lokier@cern.ch>]

Stephen C. Tweedie wrote:
> > 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.

Doesn't this also result in a swap-cache leak, or are orphan swap-cache
pages reclaimed eventually?

> > 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.

And it's even cheaper to do MADV_FREE so you skip demand-zeroing if
memory pressure doesn't require that.

-- Jamie
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