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Date:   Wed, 29 Nov 2017 15:32:08 +0900
From:   Joonsoo Kim <>
To:     Mel Gorman <>
Cc:     Johannes Weiner <>,
        Vlastimil Babka <>,
        Andrew Morton <>,,,
Subject: Re: [PATCH] mm, compaction: direct freepage allocation for async
 direct compaction

On Thu, Nov 23, 2017 at 02:08:43PM +0000, Mel Gorman wrote:

> 3. Another reason a linear scanner was used was because we wanted to
>    clear entire pageblocks we were migrating from and pack the target
>    pageblocks as much as possible. This was to reduce the amount of
>    migration required overall even though the scanning hurts. This patch
>    takes MIGRATE_MOVABLE pages from anywhere that is "not this pageblock".
>    Those potentially have to be moved again and again trying to randomly
>    fill a MIGRATE_MOVABLE block. Have you considered using the freelists
>    as a hint? i.e. take a page from the freelist, then isolate all free
>    pages in the same pageblock as migration targets? That would preserve
>    the "packing property" of the linear scanner.
>    This would increase the amount of scanning but that *might* be offset by
>    the number of migrations the workload does overall. Note that migrations
>    potentially are minor faults so if we do too many migrations, your
>    workload may suffer.
> 4. One problem the linear scanner avoids is that a migration target is
>    subsequently used as a migration source and leads to a ping-pong effect.
>    I don't know how bad this is in practice or even if it's a problem at
>    all but it was a concern at the time

IIUC, this potential advantage for a linear scanner would not be the
actual advantage in the *running* system.

Consider about following worst case scenario for "direct freepage
allocation" that "moved again" happens.


M: migration source page (the number denotes the ID of the page)
F: freepage (the number denotes the sequence in the freelist of the buddy)
_: other pages
migration scanner: move from left to right.

If migration happens with "direct freepage allocation", memory state
will be changed to:


And then, please assume that there is an another movable allocation
before another migration. It's reasonable assumption since there are
really many movable allocations in the *running* system.


If migration happens again, memory state will be:


M1 is moved twice and overall number of migration is two.

Now, think about a linear scanner. With the same scenario,
memory state will be as following.


Although "move again" doesn't happen in a linear scanner, final memory
state is the same and the same number of migration happens.

So, I think that "direct freepage allocation" doesn't suffer from such
a ping-poing effect. Am I missing something?


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