linux-kernel - Re: [PATCH mm] introduce reverse buddy concept to reduce buddy fragment

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Date:   Tue, 4 Jul 2017 12:46:23 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     zhouxianrong <zhouxianrong@...wei.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        vbabka@...e.cz, alexander.h.duyck@...el.com,
        l.stach@...gutronix.de, vdavydov.dev@...il.com, hannes@...xchg.org,
        minchan@...nel.org, npiggin@...il.com,
        kirill.shutemov@...ux.intel.com, gi-oh.kim@...fitbricks.com,
        luto@...nel.org, keescook@...omium.org, mark.rutland@....com,
        mingo@...nel.org, heiko.carstens@...ibm.com,
        iamjoonsoo.kim@....com, rientjes@...gle.com,
        ming.ling@...eadtrum.com, jack@...e.cz, ebru.akagunduz@...il.com,
        bigeasy@...utronix.de, Mi.Sophia.Wang@...wei.com,
        zhouxiyu@...wei.com, weidu.du@...wei.com, fanghua3@...wei.com,
        won.ho.park@...wei.com
Subject: Re: [PATCH mm] introduce reverse buddy concept to reduce buddy
 fragment

On Tue, Jul 04, 2017 at 01:24:14PM +0200, Michal Hocko wrote:
> On Tue 04-07-17 16:04:52, zhouxianrong wrote:
> > every 2s i sample /proc/buddyinfo in the whole test process.
> > 
> > the last about 90 samples were sampled after the test was done.
> 
> I've tried to explain to you that numbers without a proper testing
> metodology and highlevel metrics you are interested in and comparision
> to the base kernel are meaningless. I cannot draw any conclusion from
> looking at numbers you have posted. Are high order allocations cheaper
> to do with this patch? What about an averge order-0 allocation request?
> 

I have to agree. The patch is extremely complex for what it does which
is working around a limitation of the buddy allocator in general
(buddy's must be naturally aligned). There would have to be *strong*
justification that allocations fail even with compaction or a reclaim
cycle or that the latency is severely reduced -- neither which is
evident from the data presented. It would also have to be proven that
there is no overhead added in the general case to justify this so
without extensive justification for the complexity;

Naked-by: Mel Gorman <mgorman@...e.de>

> You are touching memory allocator hot paths and those are really
> sensitive to changes. It takes a lot of testing with different workloads
> to prove that no new regressions are introduced. That being said, I
> completely agree that reducing the memory fragmentation is an important
> objective but touching the page allocator and adding new branches there
> sounds like a problematic approach which would have to show _huge_
> benefits to be mergeable. Is it possible to improve khugepaged to
> accomplish the same thing?

Or if this is CMA related, a justification why alloc_contig_range cannot do
the same thing with a linear walk when the initial allocation attempt fails.

-- 
Mel Gorman
SUSE Labs