On Mon, 16 Nov 2015, Johannes Weiner wrote:

Dear Hugh,

[ sorry, I just noticed this email now ]

No problem: as you can see, I'm very far from keeping up myself.

On Wed, Oct 21, 2015 at 01:05:53PM -0700, Hugh Dickins wrote:

quoted

On Wed, 21 Oct 2015, Johannes Weiner wrote:

quoted

On Mon, Oct 19, 2015 at 02:13:01PM -0400, Johannes Weiner wrote:

quoted

mm-increase-swap_cluster_max-to-batch-tlb-flushes.patch changed
SWAP_CLUSTER_MAX from 32 pages to 256 pages, inadvertantly switching
the scan window for vmpressure detection from 2MB to 16MB. Revert.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/vmpressure.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index c5afd57..74f206b 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c

@@ -38,7 +38,7 @@
  * TODO: Make the window size depend on machine size, as we do for vmstat
  * thresholds. Currently we set it to 512 pages (2MB for 4KB pages).
  */
-static const unsigned long vmpressure_win = SWAP_CLUSTER_MAX * 16;
+static const unsigned long vmpressure_win = SWAP_CLUSTER_MAX;

Argh, Mel's patch sets SWAP_CLUSTER_MAX to 256, so this should be
SWAP_CLUSTER_MAX * 2 to retain the 512 pages scan window.

Andrew could you please update this fix in-place? Otherwise I'll
resend a corrected version.

Thanks, and sorry about that.

I don't understand why "SWAP_CLUSTER_MAX * 2" is thought better than "512".
Retaining a level of obscurity, that just bit us twice, is a good thing?

I'm not sure it is. But it doesn't seem entirely wrong to link it to
the reclaim scan window, either--at least be a multiple of it so that
the vmpressure reporting happens cleanly at the end of a scan cycle?

I don't mind changing it to 512, but it doesn't feel like an obvious
improvement, either.

Never mind: I thought it worth calling out at the time, but now that
your change is in, I've no reason to make a fuss over it - if it ever
bites us again, we can rework it then.

Besides, when I wrote, I hadn't noticed the comment there about
SWAP_CLUSTER_MAX, just out of sight before the context of your fix:
that would have to be reworded, and I don't know what it should say.

Oh, it's all still mmotm stuff, not in 4.4-rc.  Well, even so,
forget my interjection: we've all got bigger fish to fry.

Hugh

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