Messages in this thread | | | Date | Sat, 7 Jun 2008 15:56:20 -0400 | From | Rik van Riel <> | Subject | Re: [PATCH -mm 04/25] free swap space on swap-in/activation |
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On Fri, 6 Jun 2008 18:04:30 -0700 Andrew Morton <akpm@linux-foundation.org> wrote: > On Fri, 06 Jun 2008 16:28:42 -0400 > Rik van Riel <riel@redhat.com> wrote: > > From: Rik van Riel <riel@redhat.com> > > > > Free swap cache entries when swapping in pages if vm_swap_full() > > [swap space > 1/2 used]. Uses new pagevec to reduce pressure > > on locks. > > > > Signed-off-by: Rik van Riel <riel@redhat.com> > > Signed-off-by: Lee Schermerhorn <Lee.Schermerhorn@hp.com> > > Signed-off-by: MinChan Kim <minchan.kim@gmail.com> > > > > --- > > include/linux/pagevec.h | 1 + > > include/linux/swap.h | 6 ++++++ > > mm/swap.c | 18 ++++++++++++++++++ > > mm/swapfile.c | 25 ++++++++++++++++++++++--- > > mm/vmscan.c | 7 +++++++ > > 5 files changed, 54 insertions(+), 3 deletions(-) > > > > Index: linux-2.6.26-rc2-mm1/mm/vmscan.c > > =================================================================== > > --- linux-2.6.26-rc2-mm1.orig/mm/vmscan.c 2008-05-23 14:21:33.000000000 -0400 > > +++ linux-2.6.26-rc2-mm1/mm/vmscan.c 2008-05-23 14:21:33.000000000 -0400 > > @@ -619,6 +619,9 @@ free_it: > > continue; > > > > activate_locked: > > + /* Not a candidate for swapping, so reclaim swap space. */ > > + if (PageSwapCache(page) && vm_swap_full()) > > The patch puts rather a lot of pressure onto vm_swap_full(). We might > want to look into optimising that. > > - Is the 50% thing optimum? Could go higher and perhaps should be > based on amount-of-memory. > > - Can precalculate the fraction rather than doing it inline all the time.
I do not know if 50% is optimum. It is just what the upstream kernel has had since 2.4.10 or so. Before that it used to be 75%. This same percentage is used to free swap spaces at swapin time.
> - Can make total_swap_pages __read_mostly and have a think about > nr_swap_pages too.
I wonder if we wouldn't be off best simply placing the two on their own cache line. After all, they are often handled together.
I believe that should be a separate patch though, since I am not changing that situation from what is already upstream and this patch series contains more than enough stuff already.
> - Can completely optimise the thing away if !CONFIG_SWAP. > > > Has all this code been tested with CONFIG_SWAP=n?
With CONFIG_SWAP=n the macro PageSwapCache(page) will always be declared false due to the declarations in page-flags.h. That means that vm_swap_full() will be evaluated and the compiler should leave it out.
#ifdef CONFIG_SWAP PAGEFLAG(SwapCache, swapcache) #else PAGEFLAG_FALSE(SwapCache) #endif
> > +void pagevec_swap_free(struct pagevec *pvec)
> What's going on here. > > Normally we'll bump a page's refcount to account for its presence in a > pagevec. This code doesn't do that. > > Is it safe? If so, how come?
It is safe because the callers already hold an extra reference to each page. I have added a full kerneldoc comment to this function to explain that.
-- All rights reversed.
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