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The boot wrapper is currently built with -Os. By building with O2 we
can meaningfully reduce the time decompressing the kernel.
I tested by comparing 10 runs of each option in Qemu and on hardware.
The kernel is compressed with KERNEL_XZ built with GCC 8.2.0-7ubuntu1.
The values are counts of the timebase.
Qemu TCG powernv Power8:
Os O2 O3
median 10221123889 6201518438 6568186825
stddev 1361267211 429090641 657930076
improvement 39.33% 35.74%
Palmetto Power8:
Os O2 O3
median 50279 50599 35790
stddev 992144533 627130655 623721078
improvement 36.79% 37.13%
Romulus Power9:
Os O2 O3
median 670312391 454733720 448881398
stddev 157569 107276 108760
improvement 32.16% 33.03%
TCG was quite noisy, with every few runs producing an outlier. Even so,
O2 is faster than O3. On hardware the numbers were less noisy and O3 is
slightly faster than O2.
The wrapper size increases when moving from Os. Comparing zImage.epapr
to the existing Os build using bloat-o-meter:
Before=43401, After=56837 (13KB), chg +30.96%
Before=43401, After=64305 (20KB), chg +48.16%
I chose O2 for a balance between Qemu and hardware speed up.
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
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| .. | ||
| boot | ||
| configs | ||
| crypto | ||
| include | ||
| kernel | ||
| kvm | ||
| lib | ||
| math-emu | ||
| mm | ||
| net | ||
| oprofile | ||
| perf | ||
| platforms | ||
| purgatory | ||
| sysdev | ||
| tools | ||
| xmon | ||
| Kconfig | ||
| Kconfig.debug | ||
| Makefile | ||
| Makefile.postlink | ||