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

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Date:	Mon, 28 Jul 2008 17:17:28 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: PERF: performance tests with the split LRU VM in -mm

On Mon, 28 Jul 2008 20:03:11 -0400
Rik van Riel <riel@...hat.com> wrote:

> On Mon, 28 Jul 2008 19:57:13 -0400
> Rik van Riel <riel@...hat.com> wrote:
> > On Mon, 28 Jul 2008 16:41:24 -0700
> > Andrew Morton <akpm@...ux-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
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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