src/os/solaris/vm/os_solaris.cpp
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rev 6280 : 8040140: System.nanoTime() is slow and non-monotonic on OS X
Reviewed-by: sspitsyn, shade
*** 1393,1419 ****
// gethrtime can move backwards if read from one cpu and then a different cpu
// getTimeNanos is guaranteed to not move backward on Solaris
inline hrtime_t getTimeNanos() {
if (VM_Version::supports_cx8()) {
const hrtime_t now = gethrtime();
! // Use atomic long load since 32-bit x86 uses 2 registers to keep long.
! const hrtime_t prev = Atomic::load((volatile jlong*)&max_hrtime);
! if (now <= prev) return prev; // same or retrograde time;
const hrtime_t obsv = Atomic::cmpxchg(now, (volatile jlong*)&max_hrtime, prev);
assert(obsv >= prev, "invariant"); // Monotonicity
// If the CAS succeeded then we're done and return "now".
! // If the CAS failed and the observed value "obs" is >= now then
! // we should return "obs". If the CAS failed and now > obs > prv then
// some other thread raced this thread and installed a new value, in which case
// we could either (a) retry the entire operation, (b) retry trying to install now
! // or (c) just return obs. We use (c). No loop is required although in some cases
// we might discard a higher "now" value in deference to a slightly lower but freshly
! // installed obs value. That's entirely benign -- it admits no new orderings compared
// to (a) or (b) -- and greatly reduces coherence traffic.
! // We might also condition (c) on the magnitude of the delta between obs and now.
// Avoiding excessive CAS operations to hot RW locations is critical.
! // See http://blogs.sun.com/dave/entry/cas_and_cache_trivia_invalidate
return (prev == obsv) ? now : obsv ;
} else {
return oldgetTimeNanos();
}
}
--- 1393,1424 ----
// gethrtime can move backwards if read from one cpu and then a different cpu
// getTimeNanos is guaranteed to not move backward on Solaris
inline hrtime_t getTimeNanos() {
if (VM_Version::supports_cx8()) {
const hrtime_t now = gethrtime();
! if (AssumeMonotonicOSTimers) {
! return now;
! }
!
! const hrtime_t prev = max_hrtime;
! if (now <= prev) {
! return prev; // same or retrograde time;
! }
const hrtime_t obsv = Atomic::cmpxchg(now, (volatile jlong*)&max_hrtime, prev);
assert(obsv >= prev, "invariant"); // Monotonicity
// If the CAS succeeded then we're done and return "now".
! // If the CAS failed and the observed value "obsv" is >= now then
! // we should return "obsv". If the CAS failed and now > obsv > prv then
// some other thread raced this thread and installed a new value, in which case
// we could either (a) retry the entire operation, (b) retry trying to install now
! // or (c) just return obsv. We use (c). No loop is required although in some cases
// we might discard a higher "now" value in deference to a slightly lower but freshly
! // installed obsv value. That's entirely benign -- it admits no new orderings compared
// to (a) or (b) -- and greatly reduces coherence traffic.
! // We might also condition (c) on the magnitude of the delta between obsv and now.
// Avoiding excessive CAS operations to hot RW locations is critical.
! // See https://blogs.oracle.com/dave/entry/cas_and_cache_trivia_invalidate
return (prev == obsv) ? now : obsv ;
} else {
return oldgetTimeNanos();
}
}