linux-kernel - Re: [PATCHv7 10/24] zram: add zlib compression backend support

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Message-ID: <uaxb7sbvrg3eqn2sp66sg77urjzr7jwi2m2bwigvj5n5cta2xu@qsks2da3zrha>
Date: Tue, 13 May 2025 14:41:07 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Zaslonko Mikhail <zaslonko@...ux.ibm.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Minchan Kim <minchan@...nel.org>, linux-kernel@...r.kernel.org, 
	Andrew Morton <akpm@...ux-foundation.org>, Ilya Leoshkevich <iii@...ux.ibm.com>, 
	Heiko Carstens <hca@...ux.ibm.com>, linux-s390@...r.kernel.org
Subject: Re: [PATCHv7 10/24] zram: add zlib compression backend support

Sorry for the delay,

On (25/05/09 17:18), Zaslonko Mikhail wrote:
> > When zram transitioned from Crypto API (scomp) to custom compression
> > API I picked the CryptoAPI deflate DEFLATE_DEF_WINBITS value:
> > 
> > crypto/deflate.c: DEFLATE_DEF_WINBITS	11
> > 
> > which is then passed to zlib_deflateInit2() and zlib_inflateInit2().
> > 
> >> I tried to build the kernel with DEFLATE_DEF_WINBITS set to 15 and
> >> verified that s390 hardware deflate acceleration works for zram devices
> >> with a deflate compression.
> > 
> > If we define it as 15 on non-s390 machines, will there be any
> > consequences?  Increased memory usage?  By how much?
> 
> On s390, setting windowBits to 15 would lead to zlib workarea size
> increased by 120K (0x24dc8 -> 0x42dc8) per compression stream,
> i.e. per online CPU. 
> On non-s390 machine, that impact will be about 115K per stream. 
> Increasing window size should improve deflate compression,
> although the compression speed might be affected. Couldn't find any
> relevant zlib benchmarks though.
> 
> Not sure what other consequences might there be for zram. Do you see any?

The increased per-CPU memory usage is the only thing I can think of.
I guess for zram we could turn this into a run-time parameter, but for
Crypto API compile-time is the only option, I guess.

Can you send a patch series (for zram and Crypto API) that sets
windowBits to 15?