Comment 8 for bug 1840934

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);
..."