* having to inspect a "current time left" variable (e.g. $curTTL, $curTTLs), a cache
* regeneration will automatically be triggered using the callback.
*
- * The simplest way to avoid stampedes for hot keys is to use
- * the 'lockTSE' option in $opts. If cache purges are needed, also:
+ * The $ttl argument and "hotTTR" option (in $opts) use time-dependant randomization
+ * to avoid stampedes. Keys that are slow to regenerate and either heavily used
+ * or subject to explicit (unpredictable) purges, may need additional mechanisms.
+ * The simplest way to avoid stampedes for such keys is to use 'lockTSE' (in $opts).
+ * If explicit purges are needed, also:
* - a) Pass $key into $checkKeys
* - b) Use touchCheckKey( $key ) instead of delete( $key )
*
* This is useful if the source of a key is suspected of having possibly changed
* recently, and the caller wants any such changes to be reflected.
* Default: WANObjectCache::MIN_TIMESTAMP_NONE.
- * - hotTTR: Expected time-till-refresh for keys that average ~1 hit/second.
- * This should be greater than "ageNew". Keys with higher hit rates will regenerate
- * more often. This is useful when a popular key is changed but the cache purge was
- * delayed or lost. Seldom used keys are rarely affected by this setting, unless an
- * extremely low "hotTTR" value is passed in.
+ * - hotTTR: Expected time-till-refresh (TTR) for keys that average ~1 hit/second (1 Hz).
+ * Keys with a hit rate higher than 1Hz will refresh sooner than this TTR and vise versa.
+ * Such refreshes won't happen until keys are "ageNew" seconds old. The TTR is useful at
+ * reducing the impact of missed cache purges, since the effect of a heavily referenced
+ * key being stale is worse than that of a rarely referenced key. Unlike simply lowering
+ * $ttl, seldomly used keys are largely unaffected by this option, which makes it possible
+ * to have a high hit rate for the "long-tail" of less-used keys.
* Default: WANObjectCache::HOT_TTR.
* - lowTTL: Consider pre-emptive updates when the current TTL (seconds) of the key is less
* than this. It becomes more likely over time, becoming certain once the key is expired.
// A deleted key with a negative TTL left must be tombstoned
$isTombstone = ( $curTTL !== null && $value === false );
+ if ( $isTombstone && $lockTSE <= 0 ) {
+ // Use the INTERIM value for tombstoned keys to reduce regeneration load
+ $lockTSE = 1;
+ }
// Assume a key is hot if requested soon after invalidation
$isHot = ( $curTTL !== null && $curTTL <= 0 && abs( $curTTL ) <= $lockTSE );
// Use the mutex if there is no value and a busy fallback is given
if ( $lockAcquired ) {
// Avoid using delete() to avoid pointless mcrouter broadcasting
- $this->cache->changeTTL( self::MUTEX_KEY_PREFIX . $key, 1 );
+ $this->cache->changeTTL( self::MUTEX_KEY_PREFIX . $key, (int)$preCallbackTime - 60 );
}
return $value;
}
/**
- * Check if a key should be regenerated (using random probability)
+ * Check if a key is nearing expiration and thus due for randomized regeneration
*
* This returns false if $curTTL >= $lowTTL. Otherwise, the chance
* of returning true increases steadily from 0% to 100% as the $curTTL
return array_diff( $keys, $keysFound );
}
- /**
+ /**
* @param array $keys
* @param array $checkKeys
* @return array Map of (cache key => mixed)
dépôts / lhc / web / wiklou.git / blobdiff