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

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Date:   Fri, 4 Nov 2022 17:01:15 -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 Sat, Nov 05, 2022 at 08:41:37AM +0900, Sergey Senozhatsky wrote:
> On (22/11/04 10:43), Minchan Kim wrote:
> > > > Configure 2 alternative algos, with priority 1 and 2
> > > > 
> > > > 	echo "name=lz4 priority=1" > recomp_algo
> > > > 	echo "name=lz5 priority=2" > recomp_algo
> > > > 
> > > > Recompress pages using algo 1 and algo 2
> > > > 
> > > > 	echo "type=huge threshold=3000 algo_idx=1" > recompress
> > > > 	echo "type=idle threshold=2000 algo_idx=2" > recompress
> > > > 
> > > > Maybe we can even pass algo name instead of idx.
> > > 
> > > Or pass priority= so that interface that uses algorithms has the
> > > same keyword that the interface that configures those algorithms.
> > 
> > Hmm, why do we need algo_idx here if we already set up every
> > fields at algorithm setup time?
> > 
> > My understaind(assuming default(i.e., primary) algo is lzo) is
> > 
> >     echo "name=lz4 priority=1" > recomp_algo
> >     echo "name=lz5 priority=2" > recomp_algo
> > 
> >     echo "type=huge threshold=3000" > recompress
> > 
> > It will try compress every objects which greater than 3000B with lz4 first
> > and then lz5 if it's stillgreater or equal than 3000(or same size class).
> 
> One can be SW one can be HW. So I thought about having flexibility here.
> Instead of doing
> 
> 	for (idx = 1; idx < MAX_IDX; idx++) {
> 		len = zcomp_compress(zram->comps[idx]);
> 		if (len <= threshold)
> 			break;
> 	}
> 
> We would just directly use the suggested algo.
> 
> But we probably don't need that param at all and can use
> the loop instead?

I don't understand what param you are saying. I expected
the zram->comps array already has sorted algoritm based on the
priority so the loop will try compression as expected so loop is fine.
Are we on same page?

> 
> [..]
> >     echo "name=lz4 priority=1" > recomp_algo
> >     echo "name=lz5 priority=2" > recomp_algo
> >
> >     # or I realized to change lz5 to lz7 so
> >     echo "name=lz6 priority=2" > recomp_algo
> 
> So the latter should delete lz5 at idx 2 and put lz6 there?
> I can add that.

Yub.