Make updateGauge lock-free in critical paths of APC and APCng #135
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TobiasBengtsson
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When setting a value on a gauge, first apcu_store is called in updateGauge and then apcu_add is called in storeMetadata. Both of these operations takes a write lock in APCu which can cause contention. - Change updateGauge to use a compare-and-swap algorithm instead of apcu_store, unless we're adding something for the first time(s) in which case we just set it with apcu_store. - Don't call updateMetadata and storeLabelKeys unless it's the first time we're setting a gauge (matches the behaviour of inc/dec). - For extra safety, make a cheap rlock-only check in storeMetadata if the key exists before calling apcu_add with a wlock. Signed-off-by: Tobias Bengtsson <me@tobiasbengtsson.se>
When setting a value on a gauge, apcu_store is always called two times, taking a write lock each that can cause contention. - Replace apcu_store with compare-and-swap algorithm unless we store for the first time(s) - Eagerly fetch old value instead of apcu_exists, since we will anyway need it for the compare-and-swap algorithms that follows, saving one call to APCu - Make sure we never enter an infinite loop in the compare-and-swap section by falling back to a first-time insert if $old === false. A fix for the potential infinite loop below will be done in a separate commit. Signed-off-by: Tobias Bengtsson <me@tobiasbengtsson.se>
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See commit messages for more info. TL;DR
setting values on a gauge can cause high contention due to multiple APCu write locks.