linux-kernel - Re: [RFC] Analyzing zpool allocators / Removing zbud and z3fold

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Date: Thu, 22 Feb 2024 13:23:43 +0700
From: Nhat Pham <nphamcs@...il.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Vitaly Wool <vitaly.wool@...sulko.com>, 
	Miaohe Lin <linmiaohe@...wei.com>, Johannes Weiner <hannes@...xchg.org>, Linux-MM <linux-mm@...ck.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Christoph Hellwig <hch@...radead.org>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>, Minchan Kim <minchan@...nel.org>, 
	Chris Down <chris@...isdown.name>, Seth Jennings <sjenning@...hat.com>, 
	Dan Streetman <ddstreet@...e.org>, Chris Li <chrisl@...nel.org>
Subject: Re: [RFC] Analyzing zpool allocators / Removing zbud and z3fold

On Fri, Feb 9, 2024 at 10:27 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> I did not perform any sophisticated analysis on these histograms, but
> eyeballing them makes it clear that all allocators have somewhat
> similar latencies. zbud is slightly better than zsmalloc, and z3fold
> is slightly worse than zsmalloc. This corresponds naturally to the
> build times in (a).
>
> (c) Maximum size of the zswap pool
>
> *** zsmalloc ***
> 1,137,659,904 bytes = ~1.13G
>
> *** zbud ***
> 1,535,741,952 bytes = ~1.5G
>
> *** z3fold ***
> 1,151,303,680 bytes = ~1.15G
>
> zbud consumes ~32.7% more memory, and z3fold consumes ~1.8% more
> memory. This makes sense because zbud only stores a maximum of two
> compressed pages on each order-0 page, regardless of the compression
> ratio, so it is bound to consume more memory.
>
> -------------------------------- </Results> --------------------------------
>
> According to those results, it seems like zsmalloc is superior to
> z3fold in both efficiency and latency. Zbud has a small latency
> advantage, but that comes with a huge cost in terms of memory
> consumption. Moreover, most known users of zswap are currently using
> zsmalloc. Perhaps some folks are using zbud because it was the default
> allocator up until recently. The only known disadvantage of zsmalloc
> is the dependency on MMU.
>
> Based on that, I think it doesn't make sense to keep all 3 allocators
> going forward. I believe we should start with removing either zbud or
> z3fold, leaving only one allocator supporting MMU. Once zsmalloc
> supports !MMU (if possible), we can keep zsmalloc as the only
> allocator.
>
> Thoughts and feedback are highly appreciated. I tried to CC all the
> interested folks, but others feel free to chime in.

I already voiced my opinion on the other thread, but to reiterate, my
vote is towards deprecating/removing z3fold :)
Unless someone can present a convincing argument/use case/workload,
where z3fold outshines both zbud and zsmalloc, or at least is another
point on the Pareto front of (latency x memory saving).