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6784100: getTimeNanos - CAS reduction

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Get rid of the CAS loop in getTimeNanos to reduce coherence traffic on Solaris.

Reviewed-by: acorn, kvn, ysr
This commit is contained in:
Xiaobin Lu 2008-12-19 14:40:28 -08:00
parent 0f5687c1eb
commit 74d9cf0fb8
1 changed files with 18 additions and 10 deletions
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28
hotspot/src/os/solaris/vm/os_solaris.cpp
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@ -1638,16 +1638,24 @@ inline hrtime_t oldgetTimeNanos() {
// getTimeNanos is guaranteed to not move backward on Solaris
inline hrtime_t getTimeNanos() {
if (VM_Version::supports_cx8()) {
bool retry = false;
hrtime_t newtime = gethrtime();
hrtime_t oldmaxtime = max_hrtime;
hrtime_t retmaxtime = oldmaxtime;
while ((newtime > retmaxtime) && (retry == false || retmaxtime != oldmaxtime)) {
oldmaxtime = retmaxtime;
retmaxtime = Atomic::cmpxchg(newtime, (volatile jlong *)&max_hrtime, oldmaxtime);
retry = true;
}
return (newtime > retmaxtime) ? newtime : retmaxtime;
const hrtime_t now = gethrtime();
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 "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();
}