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fad153ec45
unnecessary cache resets. The major changes are: * When the queue overflows, we only issue a cache reset to the specific backend or backends that still haven't read the oldest message, rather than resetting everyone as in the original coding. * When we observe backend(s) falling well behind, we signal SIGUSR1 to only one backend, the one that is furthest behind and doesn't already have a signal outstanding for it. When it finishes catching up, it will in turn signal SIGUSR1 to the next-furthest-back guy, if there is one that is far enough behind to justify a signal. The PMSIGNAL_WAKEN_CHILDREN mechanism is removed. * We don't attempt to clean out dead messages after every message-receipt operation; rather, we do it on the insertion side, and only when the queue fullness passes certain thresholds. * Split SInvalLock into SInvalReadLock and SInvalWriteLock so that readers don't block writers nor vice versa (except during the infrequent queue cleanout operations). * Transfer multiple sinval messages for each acquisition of a read or write lock.