Re: [RFC 4/4] mm, page_alloc: disallow migratetype fallback in fastpath

[Vlastimil Babka]

On 10/13/2016 09:58 AM, Joonsoo Kim wrote:
> On Thu, Sep 29, 2016 at 11:05:48PM +0200, Vlastimil Babka wrote:
> > The previous patch has adjusted async compaction so that it helps against
> > longterm fragmentation when compacting for a non-MOVABLE high-order allocation.
> > The goal of this patch is to force such allocations go through compaction
> > once before being allowed to fallback to a pageblock of different migratetype
> > (e.g. MOVABLE). In contexts where compaction is not allowed (and for order-0
> > allocations), this delayed fallback possibility can still help by trying a
> > different zone where fallback might not be needed and potentially waking up
> > kswapd earlier.
>
> Hmm... can we justify this compaction overhead in case of that there is
> high order freepages in other migratetype pageblock? There is no guarantee
> that longterm fragmentation happens and it affects the system
> peformance.

Yeah, I hoped testing would show whether this makes any difference, and 
what the overhead is, and then we can decide whether it's worth.

> And, it would easilly fail to compact in unmovable pageblock since
> there would not be migratable pages if everything works as our
> intended. So, I guess that checking it over and over doesn't help to
> reduce fragmentation and just increase latency of allocation.

The pageblock isolation_suitable heuristics of compaction should 
mitigate rescanning blocks without success. We could also add a per-zone 
flag that gets set during a fallback allocation event and cleared by 
finished compaction, or something.

> Thanks.