Re: [PATCH] lazy freeing of memory through MADV_FREE

[Andrew Morton]

On Tue, 17 Apr 2007 03:15:51 -0400
Rik van Riel <riel@xxxxxxxxxx> wrote:

> Make it possible for applications to have the kernel free memory
> lazily.  This reduces a repeated free/malloc cycle from freeing
> pages and allocating them, to just marking them freeable.  If the
> application wants to reuse them before the kernel needs the memory,
> not even a page fault will happen.
> 
> This patch, together with Ulrich's glibc change, increases
> MySQL sysbench performance by a factor of 2 on my quad core
> test system.
> 
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> 
> ---
> Ulrich Drepper has test glibc RPMS for this functionality at:
> 
>      http://people.redhat.com/drepper/rpms
> 
> Andrew, I have stress tested this patch for a few days now and
> have not been able to find any more bugs.  I believe it is ready
> to be merged in -mm, and upstream at the next merge window.
> 
> When the patch goes upstream, I will submit a small follow-up
> patch to revert MADV_DONTNEED behaviour to what it did previously
> and have the new behaviour trigger only on MADV_FREE: at that
> point people will have to get new test RPMs of glibc.
> 
> 

I've also merged Nick's "mm: madvise avoid exclusive mmap_sem".

- Nick's patch also will help this problem.  It could be that your patch
  no longer offers a 2x speedup when combined with Nick's patch.

  It could well be that the combination of the two is even better, but it
  would be nice to firm that up a bit.  Chewing a page flag is an expensive
  thing to do.

  I do go on about that.  But we're adding page flags at about one per
  year, and when we run out we're screwed - we'll need to grow the
  pageframe.

- I need to update your patch for Nick's patch.  Please confirm that
  down_read(mmap_sem) is sufficient for MADV_FREE.


Stylistic nit:

> +	if (PageLazyFree(page) && !migration) {
> +		/* There is new data in the page.  Reinstate it. */
> +		if (unlikely(pte_dirty(pteval))) {
> +			set_pte_at(mm, address, pte, pteval);
> +			ret = SWAP_FAIL;
> +			goto out_unmap;
> +		}

The comment should be inside the second `if' statement.  As it is, It
looks like we reinstate the page if (PageLazyFree(page) && !migration).

-
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