Currently the VDSO does not handle
clock_gettime( CLOCK_MONOTONIC_RAW, &ts )
on Intel / AMD - it calls
for this clock, which issues a syscall, having an unacceptably high
latency (minimum measurable time or time between measurements)
of 300-700ns on 2 2.8-3.9ghz Haswell x86_64 Family'_'Model : 06_3C
machines under various versions of Linux.
Sometimes, particularly when correlating elapsed time to performance
counter values, user-space code needs to know elapsed time from the
perspective of the CPU no matter how "hot" / fast or "cold" / slow it
might be running wrt NTP / PTP "real" time; when code needs this,
the latencies associated with a syscall are often unacceptably high.
I reported this as Bug #198161 :
and in previous posts with subjects matching 'CLOCK_MONOTONIC_RAW' .
This patch handles CLOCK_MONOTONIC_RAW clock_gettime() in the VDSO ,
by exporting the raw clock calibration, last cycles, last xtime_nsec,
and last raw_sec value in the vsyscall_gtod_data during vsyscall_update() .
Now the new do_monotonic_raw() function in the vDSO has a latency of @ 24ns
on average, and the test program:
succeeds with arguments: '-c 4 -t 120' or any arbitrary -t value.
The patch is against Linus' latest 4.16-rc5 tree,
current HEAD of :
This patch affects only files:
There are 3 patches in the series :
Patch #1 makes the VDSO handle clock_gettime(CLOCK_MONOTONIC_RAW) with
Patch #2 makes the VDSO handle clock_gettime(CLOCK_MONOTONIC_RAW) with a new
rdtscp() function in msr.h
Patch #3 makes the VDSO export TSC calibration data via a new function in
unsigned int __vdso_linux_tsc_calibration ( struct
linux_tsc_calibration *tsc_cal )
that user code can optionally call.
Patches #2 & #3 should be considered "optional" .
Patch #2 makes clock_gettime(CLOCK_MONOTONIC_RAW) calls have @ half the
of clock_gettime(CLOCK_MONOTONIC) calls.
I think something like Patch #3 is necessary to export TSC calibration data
to user-space TSC readers.
Jason Vas Dias