linux-kernel - Re: [PATCHv4 2/9] zram: Add recompression algorithm sysfs knob

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Date:   Thu, 3 Nov 2022 09:34:51 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Nitin Gupta <ngupta@...are.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCHv4 2/9] zram: Add recompression algorithm sysfs knob

On Thu, Nov 03, 2022 at 12:05:06PM +0900, Sergey Senozhatsky wrote:

< snip >

> > Just open question(I might be too paranoid?)
> > 
> > I am thinking someone want to add third comp algorithm in future
> > to balance decompression and memory efficiency.
> > 
> > If it's not too crazy idea, let's think about the interface.
> > Maybe, could we make the recomp knobs works like list?
> > 
> > # A primary comp
> > echo "A" > /zram/comp_algo
> > 
> > # Multiple secondary comps
> > echo "B threshold" > /zram/add_recomp_algo
> > echo "C threshold" > /zram/add_recomp_algo
> > echo "D threshold" > /zram/add_recomp_algo
> 
> What is the threshold here? My design approach is that ZRAM doesn't do

As your term, watermark but yeah, priority you suggested would be good
for me.

> recompression on its own, so no magic is happening automatically. It's
> the user-space that triggers recompression for selected pages when
> user-space thinks it's time to. This allows us to have various flexible
> policies and consider things that ZRAM is not even aware of: battery level,
> free memory, CPU load average, etc. E.g. no recompression when all CPUs
> are busy rendering video game, or when we are draining battery too fast,
> etc.
> 
> > "cat /zram/recomp_algo" shows the list
> > 
> > echo "C" > /zram/remove_recomp_algo
> > will remove the C algorithm in stack.
> 
> What is the use case for removal of a secondary algorithm?

Without the interface, How can we modify the selection if admin want to
change the order of second algorithms?

> 
> > My point is that we don't need to implement it atm but makes the
> > interface to open the possibility for future extension.
> > 
> > What do you think?
> 
> So, as far as I understand, we don't have reason to add remove_recomp_algo
> right now. And existing recomp_algo does not enforce any particular format,
> it can be extended. Right now we accept "$name" but can do something like
> "$name:$priority". The only thing that we probably need to do is rename
> recomp_algo to either add_recomp_algo or register_recomp_algo?

Yeah, I like the name and priority format.

Only question is how we could support algorithm selection change
under considering multiple secondary algorithms.