Re: [PATCH v2 2/2] kbuild: pass --stream-size --no-content-size to zstd
From: Masahiro Yamada <masahiroy@kernel.org>
Date: 2021-12-05 22:53:37
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On Thu, Nov 25, 2021 at 12:30 AM Alex Xu (Hello71) [off-list ref] 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) <redacted>
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 [off-list ref]
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.
quoted hunk ↗ jump to hunk
--- 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