linux-kernel - Re: [mm PATCH v5 0/7] Deferred page init improvements

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Date:   Wed, 14 Nov 2018 16:07:42 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvdimm@...ts.01.org, davem@...emloft.net,
        pavel.tatashin@...rosoft.com, mingo@...nel.org,
        kirill.shutemov@...ux.intel.com, dan.j.williams@...el.com,
        dave.jiang@...el.com, rppt@...ux.vnet.ibm.com, willy@...radead.org,
        vbabka@...e.cz, khalid.aziz@...cle.com, ldufour@...ux.vnet.ibm.com,
        mgorman@...hsingularity.net, yi.z.zhang@...ux.intel.com
Subject: Re: [mm PATCH v5 0/7] Deferred page init improvements

On Mon 05-11-18 13:19:25, Alexander Duyck wrote:
> This patchset is essentially a refactor of the page initialization logic
> that is meant to provide for better code reuse while providing a
> significant improvement in deferred page initialization performance.
> 
> In my testing on an x86_64 system with 384GB of RAM and 3TB of persistent
> memory per node I have seen the following. In the case of regular memory
> initialization the deferred init time was decreased from 3.75s to 1.06s on
> average. For the persistent memory the initialization time dropped from
> 24.17s to 19.12s on average. This amounts to a 253% improvement for the
> deferred memory initialization performance, and a 26% improvement in the
> persistent memory initialization performance.
> 
> I have called out the improvement observed with each patch.

I have only glanced through the code (there is a lot of the code to look
at here). And I do not like the code duplication and the way how you
make the hotplug special. There shouldn't be any real reason for that
IMHO (e.g. why do we init pfn-at-a-time in early init while we do
pageblock-at-a-time for hotplug). I might be wrong here and the code
reuse might be really hard to achieve though.

I am also not impressed by new iterators because this api is quite
complex already. But this is mostly a detail.

Thing I do not like is that you keep microptimizing PageReserved part
while there shouldn't be anything fundamental about it. We should just
remove it rather than make the code more complex. I fell more and more
guilty to add there actually.
-- 
Michal Hocko
SUSE Labs