[PATCH 0/4] clock_gettime_ns and x86-64 optimizations

From: Andy Lutomirski
Date: Sun Dec 25 2011 - 11:51:21 EST

On x86-64, clock_gettime is so fast that the overhead converting to and
from nanoseconds is non-negligible. clock_gettime_ns is a different and
faster interface.

Patch 1 adds the syscall and wires it up on x86-64. Patch 2 implements
the corresponding vdso entry on x86-64. Patch 3 optimizes the vdso
call, and patch 4 is a trivial change that speeds up the vdso
clock_gettime and clock_gettime_ns implementations.

The vdso timings are (on an 800MHz Sandy Bridge mobile):

Basic implementation:

realtime 77.4ns
monotonic 79.2ns
realtime_coarse 18.1ns
monotonic_coarse 22.0ns

realtime_ns 84.9ns
monotonic_ns 85.1ns
realtime_coarse_ns 19.49
monotonic_coarse_ns 27.32

Optimized implementation:

realtime 78.5ns
monotonic 77.4ns [a little faster -- maybe significant]
realtime_coarse 18.4ns
monotonic_coarse 19.4ns

realtime_ns 77.85ns [a nice improvement]
monotonic_ns 77.75ns [ditto]
realtime_coarse_ns 18.2ns
monotonic_coarse_ns 18.2ns [a lot faster]

Inlined (patch 4): [everything is improved]

realtime 73.4ns
monotonic 72.1ns
realtime_coarse 13.2ns
monotonic_coarse 15.8ns

realtime_ns 73.15ns
monotonic_ns 72.1ns
realtime_coarse_ns 14.1ns
monotonic_coarse_ns 15.6ns

This being the middle of the holidays, I reserve the right to have made

For the git-inclined, this series is at

Andy Lutomirski (4):
Add clock_gettime_ns syscall
x86-64: Add __vdso_clock_gettime_ns vsyscall
x86-64: Optimize vdso clock_gettime
x86-64: Inline vdso clock_gettime helpers

arch/x86/include/asm/unistd_64.h | 2 +
arch/x86/include/asm/vgtod.h | 21 ++++--
arch/x86/kernel/vsyscall_64.c | 25 +++++++-
arch/x86/vdso/vclock_gettime.c | 136 ++++++++++++++++++++++++++------------
arch/x86/vdso/vdso.lds.S | 7 ++
include/linux/syscalls.h | 3 +
include/linux/time.h | 5 ++
kernel/posix-timers.c | 30 ++++++++
8 files changed, 179 insertions(+), 50 deletions(-)


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