linux-kernel - Re: [PATCHv4 0/9] zsmalloc/zram: configurable zspage size

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Date:   Tue, 15 Nov 2022 16:59:29 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nitin Gupta <ngupta@...are.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCHv4 0/9] zsmalloc/zram: configurable zspage size

On (22/11/15 15:01), Sergey Senozhatsky wrote:
> On (22/11/11 09:03), Minchan Kim wrote:
> [..]
> > for class in classes:
> >     wasted_bytes += class->pages_per_zspage * PAGE_SIZE - an object size
> > 
> > with *aggressive zpage compaction*. Now, we are relying on shrinker
> > (it might be already enough) to trigger but we could change the policy 
> > wasted memory in the class size crossed a threshold
> 
> Compaction does something good only when we can release zspage in the
> end. Otherwise we just hold the global pool->lock (assuming that we
> land zsmalloc writeback series) and simply move objects around zspages.
> So ability to limit zspage chain size still can be valuable, on another
> level, as a measure to reduce dependency on compaction success.
> 
> We may be can make compaction slightly more successful. For instance,
> if we would start move objects not only within zspages of the same size
> class, but, for example, move objects to class size + X (upper size
> classes). As an example, when all zspages in class are almost full,
> but class size + 1 has almost empty pages. In other words sort of as
> is those classes had been merged. (virtual merge). Single pool->look
> would be handy for it.

What I'm trying to say here is that "aggressiveness of compaction"
probably should be measured not by compaction frequency, but by overall
cost of compaction operations.

Aggressive frequency of compaction doesn't help us much if the state of
the pool doesn't change significantly between compactions. E.g. if we do
10 compaction calls, then only the first one potentially compacts some
zspages, the remaining ones don't do anything.

Cost of compaction operations is a measure of how hard compaction tries.
Does it move object to neighbouring classes and so on? May be we can do
something here.

But then the question is - how do we control that we don't drain battery
too fast? And perhaps some other questions too.