Re: [PATCH v2 2/2] kbuild: pass --stream-size --no-content-size to zstd

[Nick Terrell <terrelln@fb.com>]

> On Dec 5, 2021, at 2:52 PM, Masahiro Yamada <masahiroy@kernel.org> wrote:
> 
> 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.

This will definitely work from a zstd perspective. All versions
of zstd will downsize their memory usage to match the file size.

Best,
Nick Terrell

>> ---
>> 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