>From "Comment bridged from LTC Bugzilla" in a bug discussion:
"FWIW, I verified this on z14, and there clearly lz4 is (as expected)
the fastest decompression algorithm.
With vanilla 5.3-rc6 and defconfig I get the following kernel uncompression
times:
lzo: 27us
lz4: 24us
An initrd (uncompressed size ~55MB) gets these uncompression times:
lzo: 62us
lz4: 49us
So I'd clearly vote to switch to lz4 on s390 as well."
Also:
"I also instrumented the kernel code to only measure the time to decompress the
kernel. If its stckf or stcke doesn't matter in this case.
Note that if you shift a tod clock value 12 bits to the right will give you
microseconds. (All numbers I posted were actually milliseconds not microseconds
by the way).
I measured both runs (z13 + z14) when running within z/VM and IPL'ed
from the punch card reader.
Times used for decompressing the initrd were just extracted from dmesg;
no kernel instrumentation required here, since there are two messages
provided before and after initrd decompression.
Find below an extract of the patch to measure decompression time.
diff --git a/arch/s390/boot/startup.c b/arch/s390/boot/startup.c
index 7b0d054..cee3d97 100644
--- a/arch/s390/boot/startup.c
+++ b/arch/s390/boot/startup.c
@@ -146,7 +146,10 @@ void startup_kernel(void)
}
if (!IS_ENABLED(CONFIG_KERNEL_UNCOMPRESSED)) {
+ start = get_tod_clock();
img = decompress_kernel();
+ end = get_tod_clock();
+ time = (end - start) >> 12;
memmove((void *)vmlinux.default_lma, img, vmlinux.image_size);
} else if (__kaslr_offset)
memcpy((void *)vmlinux.default_lma, img, vmlinux.image_size);
..."
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https://bugs.launchpad.net/bugs/1840934
Title:
Change kernel compression method to improve boot speed
Status in linux package in Ubuntu:
Fix Released
Bug description:
Colin King has done some analysis of kernel boot speed using different
kernel compression methods. Results for x86 are at:
https://kernel.ubuntu.com/~cking/boot-speed-eoan-5.3/kernel-compression-method.txt
https://kernel.ubuntu.com/~cking/boot-speed-eoan-5.3/boot-speed-compression-5.3-rc4.ods
Testing of s390 gave the following:
GZIP 31528972
LZ4 192348049
LZO 85990145
From Colin: "I used the monotonic TOD timer using the stckf opcode to
fetch a 64 bit time value. Not sure how this maps to 'real time' in
seconds."
Conclusion: We should switch x86 to LZ4 and s390 to LZO. PPC and ARM
do not support LZO or LZ4, so we will stick with gzip there.
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1840934/+subscriptions
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