[OpenWrt-Devel] [PATCH 2/2] build: compress kernel debuginfo using zstd

[Matthias Schiffer]

zstd with its default settings (compression level -3) compresses better
than bzip2 -9 (which is the default setting), and is an order of magnitude
faster.

I made the following measurements for the most common compression tools
(all standard Debian Buster versions, default flags unless noted
otherwise), using the debug information of a large x86-64 kernel with
ALL_KMODS:

* kernel-debug.tar: 376M
* kernel-debug.tar.gz: 101M, compressed in ~12s
* kernel-debug.tar.bz2: 91M, compressed in ~15s
* kernel-debug.tar.xz: 57M, compressed in ~101s
* kernel-debug.tar.zst: 86M, compressed in ~1s

With zstd, there is still some room for improvement by increasing the
compression, but the slight increase in compression ratio
(22.83% -> 19.46%) does not justify the significant increase in
compression time (about 5 times on my machine) in my opinion.

Note that multithreaded compression (-T argument) does not affect
reproducibility with zstd.

Signed-off-by: Matthias Schiffer <mschiffer at universe-factory.net>
---
 include/kernel-build.mk | 2 +-
 tools/Makefile          | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/kernel-build.mk b/include/kernel-build.mk
index c371e78ab9b0..32c91a5b8359 100644
--- a/include/kernel-build.mk
+++ b/include/kernel-build.mk
@@ -70,7 +70,7 @@ ifdef CONFIG_COLLECT_KERNEL_DEBUG
 	$(FIND) $(KERNEL_BUILD_DIR)/debug -type f | $(XARGS) $(KERNEL_CROSS)strip --only-keep-debug
 	$(TAR) c -C $(KERNEL_BUILD_DIR) debug \
 		$(if $(SOURCE_DATE_EPOCH),--mtime="@$(SOURCE_DATE_EPOCH)") \
-		| bzip2 -c -9 > $(BIN_DIR)/kernel-debug.tar.bz2
+		| zstd -T0 -f -o $(BIN_DIR)/kernel-debug.tar.zst
   endef
 endif
 
diff --git a/tools/Makefile b/tools/Makefile
index 02556d4b83f3..169f36c5bb69 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -37,6 +37,7 @@ tools-y += lzma squashfskit4 zip
 tools-$(BUILD_B43_TOOLS) += b43-tools
 tools-$(BUILD_ISL) += isl
 tools-$(CONFIG_USE_SPARSE) += sparse
+tools-$(CONFIG_COLLECT_KERNEL_DEBUG) += zstd
 tools-$(CONFIG_TARGET_apm821xx)$(CONFIG_TARGET_gemini) += genext2fs
 tools-$(CONFIG_TARGET_tegra) += cbootimage cbootimage-configs
 
-- 
2.26.2


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