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

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Message-ID: <20170704112414.GA14727@dhcp22.suse.cz>
Date:   Tue, 4 Jul 2017 13:24:14 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     zhouxianrong <zhouxianrong@...wei.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, vbabka@...e.cz,
        alexander.h.duyck@...el.com, mgorman@...e.de,
        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 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?

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?
-- 
Michal Hocko
SUSE Labs