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SubjectRe: [PATCH v2 2/2] kbuild: pass --stream-size --no-content-size to zstd
On Thu, Nov 25, 2021 at 12:30 AM Alex Xu (Hello71) <alex_y_xu@yahoo.ca> wrote:
>
> Otherwise, it allocates 2 GB of memory at once. Even though the majority
> of this memory is never touched, the default heuristic overcommit
> refuses this request if less than 2 GB of RAM+swap is currently
> available. This results in "zstd: error 11 : Allocation error : not
> enough memory" and the kernel failing to build.
>
> When the size is specified, zstd will reduce the memory request
> appropriately. For typical kernel sizes of ~32 MB, the largest mmap
> request will be reduced to 512 MB, which will succeed on all but the
> smallest devices.
>
> For inputs around this size, --stream-size --no-content-size may
> slightly decrease the compressed size, or slightly increase it:
> https://github.com/facebook/zstd/issues/2848.
>
> Signed-off-by: Alex Xu (Hello71) <alex_y_xu@yahoo.ca>




The reason why we need this workaround is just because we do
"cat and compress". zstd must allocate a huge memory beforehand
since it cannot predict how long the stream it will receive.

If zstd is given with a file name, it can fstat it to know its file size
and allocate the minimal amount of memory.


This is my test.
I used 'ulimit' to set the upper limit of the memory the zstd can use.


[test steps]

# Create a 1kB file
$ truncate --size=1k dummy

# Set the memory size limit to 10MB
$ ulimit -S -v 10240

# Pass the file as a argument; success
$ zstd -19 -o dummy.zst dummy
dummy : 2.15% ( 1024 => 22 bytes, dummy.zst)

# cat and zstd; fail
$ cat dummy | zstd -19 > dummy.zst
zstd: error 11 : Allocation error : not enough memory

# cat and zstd --stream-size; success
$ cat dummy | zstd -19 --stream-size=1024 > dummy.zst




scripts/Makefile.modinst was written in such a way
that zstd can know the file size by itself.

cmd_zstd = $(ZSTD) -T0 --rm -f -q $<


We cannot rewrite scripts/Makefile.lib in that way because
arch/x86/boot/compress/Makefile concatenates two files before
compression. And this is the only use-case of this feature.

So, I am seriously considering to revert this commit:

commit d3dd3b5a29bb9582957451531fed461628dfc834
Author: H. Peter Anvin <hpa@zytor.com>
Date: Tue May 5 21:17:15 2009 -0700

kbuild: allow compressors (gzip, bzip2, lzma) to take multiple inputs




With that commit reverted, zstd will take a single input file,
and we can do "zstd -o <output> <input>".


So, I will take some time to investigate that approach.




> ---
> scripts/Makefile.lib | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index ca901814986a..c98a82ca38e6 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -466,12 +466,20 @@ quiet_cmd_xzmisc = XZMISC $@
> # single pass, so zstd doesn't need to allocate a window buffer. When streaming
> # decompression is used, like initramfs decompression, zstd22 should likely not
> # be used because it would require zstd to allocate a 128 MB buffer.
> +#
> +# --stream-size to reduce zstd memory usage (otherwise zstd -22 --ultra
> +# allocates, but does not use, 2 GB) and potentially improve compression.
> +#
> +# --no-content-size to save three bytes which we do not use (we use size_append).
> +
> +# zstd --stream-size is only supported since 1.4.4
> +zstd_stream_size = $(shell $(ZSTD) -1c --stream-size=0 --no-content-size </dev/null >/dev/null 2>&1 && printf '%s' '--stream-size=$(total_size) --no-content-size')
>
> quiet_cmd_zstd = ZSTD $@
> - cmd_zstd = { cat $(real-prereqs) | $(ZSTD) -19; $(size_append); } > $@
> + cmd_zstd = { cat $(real-prereqs) | $(ZSTD) $(zstd_stream_size) -19; $(size_append); } > $@
>
> quiet_cmd_zstd22 = ZSTD22 $@
> - cmd_zstd22 = { cat $(real-prereqs) | $(ZSTD) -22 --ultra; $(size_append); } > $@
> + cmd_zstd22 = { cat $(real-prereqs) | $(ZSTD) $(zstd_stream_size) -22 --ultra; $(size_append); } > $@
>
> # ASM offsets
> # ---------------------------------------------------------------------------
> --
> 2.34.0
>


--
Best Regards
Masahiro Yamada

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