| Message ID | 20210721115153.28620-1-ddiss@suse.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [RESEND,1/3] initramfs: move unnecessary memcmp from hot path | expand |
Ping, any feedback on this change?
I think it's a no brainer, but for kicks I ran a few unrealistic micro
benchmarks on my laptop. Extraction time for a cpio image with 1M+
directories improved by 5ms (pre: 14.614s, post: 14.609s), when averaged
across 20 runs of:
qemu-system-x86_64 -machine accel=kvm -smp cpus=1 -m 10240 \
-kernel ~/linux/arch/x86/boot/bzImage \
-initrd ./initrds/gen_cpio.out \
-append "initramfs_async=0 console=ttyS0 panic=0" -nographic \
| awk '/Trying to unpack rootfs/ {start_ts = $2};
/Freeing initrd memory/ {end_ts = $2}
END {printf "%f\n", end_ts - start_ts}'
Cheers, David
On Wed, 21 Jul 2021 13:51:51 +0200, David Disseldorp wrote:
> do_header() is called for each cpio entry and first checks for "newc"
> magic before parsing further. The magic check includes a special case
> error message if POSIX.1 ASCII (cpio -H odc) magic is detected. This
> special case POSIX.1 check needn't be done in the hot path, so move it
> under the non-newc-magic error path.
>
> Signed-off-by: David Disseldorp <ddiss@suse.de>
> ---
> init/initramfs.c | 9 ++++-----
> 1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/init/initramfs.c b/init/initramfs.c
> index af27abc59643..f01590cefa2d 100644
> --- a/init/initramfs.c
> +++ b/init/initramfs.c
> @@ -256,12 +256,11 @@ static int __init do_collect(void)
>
> static int __init do_header(void)
> {
> - if (memcmp(collected, "070707", 6)==0) {
> - error("incorrect cpio method used: use -H newc option");
> - return 1;
> - }
> if (memcmp(collected, "070701", 6)) {
> - error("no cpio magic");
> + if (memcmp(collected, "070707", 6) == 0)
> + error("incorrect cpio method used: use -H newc option");
> + else
> + error("no cpio magic");
> return 1;
> }
> parse_header(collected);
On Wed, Aug 04, 2021 at 11:31:29AM +0200, David Disseldorp wrote: > Ping, any feedback on this change? > > I think it's a no brainer, but for kicks I ran a few unrealistic micro > benchmarks on my laptop. Extraction time for a cpio image with 1M+ > directories improved by 5ms (pre: 14.614s, post: 14.609s), when averaged > across 20 runs of: > qemu-system-x86_64 -machine accel=kvm -smp cpus=1 -m 10240 \ > -kernel ~/linux/arch/x86/boot/bzImage \ > -initrd ./initrds/gen_cpio.out \ > -append "initramfs_async=0 console=ttyS0 panic=0" -nographic \ > | awk '/Trying to unpack rootfs/ {start_ts = $2}; > /Freeing initrd memory/ {end_ts = $2} > END {printf "%f\n", end_ts - start_ts}' What was the dispersion for those runs?
On Wed, 4 Aug 2021 12:57:16 +0000, Al Viro wrote: > On Wed, Aug 04, 2021 at 11:31:29AM +0200, David Disseldorp wrote: > > Ping, any feedback on this change? > > > > I think it's a no brainer, but for kicks I ran a few unrealistic micro > > benchmarks on my laptop. Extraction time for a cpio image with 1M+ > > directories improved by 5ms (pre: 14.614s, post: 14.609s), when averaged > > across 20 runs of: > > qemu-system-x86_64 -machine accel=kvm -smp cpus=1 -m 10240 \ > > -kernel ~/linux/arch/x86/boot/bzImage \ > > -initrd ./initrds/gen_cpio.out \ > > -append "initramfs_async=0 console=ttyS0 panic=0" -nographic \ > > | awk '/Trying to unpack rootfs/ {start_ts = $2}; > > /Freeing initrd memory/ {end_ts = $2} > > END {printf "%f\n", end_ts - start_ts}' > > What was the dispersion for those runs? Too high for the 5ms to be considered statistically significant. Std deviations were pre: 171ms, post: 214ms... <sigh> I'll redo this on a proper test rig. Cheers, David
diff --git a/init/initramfs.c b/init/initramfs.c index af27abc59643..f01590cefa2d 100644 --- a/init/initramfs.c +++ b/init/initramfs.c @@ -256,12 +256,11 @@ static int __init do_collect(void) static int __init do_header(void) { - if (memcmp(collected, "070707", 6)==0) { - error("incorrect cpio method used: use -H newc option"); - return 1; - } if (memcmp(collected, "070701", 6)) { - error("no cpio magic"); + if (memcmp(collected, "070707", 6) == 0) + error("incorrect cpio method used: use -H newc option"); + else + error("no cpio magic"); return 1; } parse_header(collected);
do_header() is called for each cpio entry and first checks for "newc" magic before parsing further. The magic check includes a special case error message if POSIX.1 ASCII (cpio -H odc) magic is detected. This special case POSIX.1 check needn't be done in the hot path, so move it under the non-newc-magic error path. Signed-off-by: David Disseldorp <ddiss@suse.de> --- init/initramfs.c | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-)