linux-kernel - Re: [PATCH 0/3] Chunk Heap Support on DMA-HEAP

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Message-ID: <20200819132204.2rki3xsczdy2cpsk@DESKTOP-E1NTVVP.localdomain>
Date:   Wed, 19 Aug 2020 14:22:04 +0100
From:   Brian Starkey <brian.starkey@....com>
To:     Cho KyongHo <pullip.cho@...sung.com>
Cc:     Hyesoo Yu <hyesoo.yu@...sung.com>, sumit.semwal@...aro.org,
        minchan@...nel.org, akpm@...ux-foundation.org,
        iamjoonsoo.kim@....com, joaodias@...gle.com, linux-mm@...ck.org,
        surenb@...gle.com, vbabka@...e.cz, afd@...com,
        benjamin.gaignard@...aro.org, lmark@...eaurora.org,
        labbott@...hat.com, john.stultz@...aro.org,
        christian.koenig@....com, linux-media@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
        linux-kernel@...r.kernel.org, robh+dt@...nel.org,
        devicetree@...r.kernel.org, nd@....com
Subject: Re: [PATCH 0/3] Chunk Heap Support on DMA-HEAP

Hi KyongHo,

On Wed, Aug 19, 2020 at 12:46:26PM +0900, Cho KyongHo wrote:
> I have seriously considered CPA in our product but we developed our own
> because of the pool in CPA.

Oh good, I'm glad you considered it :-)

> The high-order pages are required by some specific users like Netflix
> app. Moreover required number of bytes are dramatically increasing
> because of high resolution videos and displays in these days.
> 
> Gathering lots of free high-order pages in the background during
> run-time means reserving that amount of pages from the entier available
> system memory. Moreover the gathered pages are soon reclaimed whenever
> the system is sufferring from memory pressure (i.e. camera recording,
> heavy games).

Aren't these two things in contradiction? If they're easily reclaimed
then they aren't "reserved" in any detrimental way. And if you don't
want them to be reclaimed, then you need them to be reserved...

The approach you have here assigns the chunk of memory as a reserved
CMA region which the kernel is going to try not to use too - similar
to the CPA pool.

I suppose it's a balance depending on how much you're willing to wait
for migration on the allocation path. CPA has the potential to get you
faster allocations, but the downside is you need to make it a little
more "greedy".

Cheers,
-Brian