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

Sergey Senozhatsky
2025-05-14T02:56:43Z

On (25/05/13 14:58), Zaslonko Mikhail wrote:
> On 13.05.2025 07:41, Sergey Senozhatsky wrote:
> > 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.
>
> With 'run-time parameter' you mean adding 'windowBits' as another deflate compression
> algorithm parameter for zram? Guess we could do this, using default value of 15 then.

I sent a simple patch set [1] that adds deflate.winbits parameter
support, so that it can be configured at runtime. E.g.:

echo "priority=1 deflate.winbits=15" > /sys/block/zram0/algorithm_params

Please take a look.

> > Can you send a patch series (for zram and Crypto API) that sets
> > windowBits to 15?
>
> I can do it for zram. Not sure if Crypto should be changed as well. Or is it
> supposed to have the same compression defaults as zram?

Crypto API uses a hard-coded value of -11, which wouldn't work in your
case. However, if you don't use crypto API (e.g. zswap) in your setup
then it probably doesn't need to be patched. Cc-ed Herbert from the
crypto API side, just in case.

[1] https://lore.kernel.org/linux-kernel/20250514024825.1745489-1-senozhatsky@chromium.org