Re: PERF: performance tests with the split LRU VM in -mm

Lee Schermerhorn
2008-07-29T00:46:46Z

On Mon, 2008-07-28 at 17:17 -0700, Andrew Morton wrote:
> On Mon, 28 Jul 2008 20:03:11 -0400
> Rik van Riel <riel@redhat.com> wrote:
>
> > On Mon, 28 Jul 2008 19:57:13 -0400
> > Rik van Riel <riel@redhat.com> wrote:
> > > On Mon, 28 Jul 2008 16:41:24 -0700
> > > Andrew Morton <akpm@linux-foundation.org> wrote:
> > >
> > > > > Andrew, what is your preference between:
> > > > > http://lkml.org/lkml/2008/7/15/465
> > > > > and
> > > > > http://marc.info/?l=linux-mm&m=121683855132630&w=2
> > > > >
> > > >
> > > > Boy. They both seem rather hacky special-cases. But that doesn't mean
> > > > that they're undesirable hacky special-cases. I guess the second one
> > > > looks a bit more "algorithmic" and a bit less hacky-special-case. But
> > > > it all depends on testing..
> > >
> > > I prefer the second one, since it removes the + 1 magic (at least,
> > > for the higher priorities), instead of adding new magic like the
> > > other patch does.
> >
> > Btw, didn't you add that "+ 1" originally early on in the 2.6 VM?
>
> You mean this?
>
> /*
> * Add one to nr_to_scan just to make sure that the kernel
> * will slowly sift through the active list.
> */
> zone->nr_scan_active +=
> (zone_page_state(zone, NR_ACTIVE) >> priority) + 1;
>
>
> > Do you remember its purpose?
>
> erm, not specifically, but I tended to lavishly describe changes like
> this in the changelogging.
>
> > Does it still make sense to have that "+ 1" in the split LRU VM?
> >
> > Could we get away with just removing it unconditionally?
>
> We should do the necessary git dumpster-diving before tossing out
> hard-won changes. Otherwise we might need to spend a year
> re-discovering and re-fixing already-discovered-and-fixed things.
>
> That code has been there in one way or another for some time.
>
> In June 2004, 385c0449 did this:
>
> /*
> - * Try to keep the active list 2/3 of the size of the cache. And
> - * make sure that refill_inactive is given a decent number of pages.
> - *
> - * The "scan_active + 1" here is important. With pagecache-intensive
> - * workloads the inactive list is huge, and `ratio' evaluates to zero
> - * all the time. Which pins the active list memory. So we add one to
> - * `scan_active' just to make sure that the kernel will slowly sift
> - * through the active list.
> + * Add one to `nr_to_scan' just to make sure that the kernel will
> + * slowly sift through the active list.
> */
> - if (zone->nr_active >= 4*(zone->nr_inactive*2 + 1)) {
> - /* Don't scan more than 4 times the inactive list scan size */
> - scan_active = 4*scan_inactive;
>
> (there was some regrettable information loss there).
>
> Is the scenario which that fix addresses no longer possible?
>
>
> On a different topic, I am staring in frustration at
> introduce-__get_user_pages.patch, which says:
>
> New munlock processing need to GUP_FLAGS_IGNORE_VMA_PERMISSIONS.
> because current get_user_pages() can't grab PROT_NONE pages theresore
> it cause PROT_NONE pages can't munlock.
>
> could someone please work out for me which of these patches:
>
> vmscan-move-isolate_lru_page-to-vmscanc.patch
> vmscan-use-an-indexed-array-for-lru-variables.patch
> swap-use-an-array-for-the-lru-pagevecs.patch
> vmscan-free-swap-space-on-swap-in-activation.patch
> define-page_file_cache-function.patch
> vmscan-split-lru-lists-into-anon-file-sets.patch
> vmscan-second-chance-replacement-for-anonymous-pages.patch
> vmscan-fix-pagecache-reclaim-referenced-bit-check.patch
> vmscan-add-newly-swapped-in-pages-to-the-inactive-list.patch
> more-aggressively-use-lumpy-reclaim.patch
> pageflag-helpers-for-configed-out-flags.patch
> unevictable-lru-infrastructure.patch
> unevictable-lru-page-statistics.patch
> ramfs-and-ram-disk-pages-are-unevictable.patch
> shm_locked-pages-are-unevictable.patch
> mlock-mlocked-pages-are-unevictable.patch

Andrew:

Kosaki-san's patch to introduce __get_user_pages() is a patch to the
above unevictable, mlocked pages. He enhanced get_user_pages() so that
we could fault in PROT_NONE pages for munlocking, to replace the page
table walker [subsequent patches in that series]. He replaced the page
table walker to avoid the "sleeping while atomic" for 32-bit/HIGHPTE
configs.

Lee

> mlock-downgrade-mmap-sem-while-populating-mlocked-regions.patch
> mmap-handle-mlocked-pages-during-map-remap-unmap.patch
>
> that patch fixes?
>
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