Aaron Schulz has uploaded a new change for review. (
https://gerrit.wikimedia.org/r/386113 )
Change subject: Add more hotTTR comments to WANObjectCache
......................................................................
Add more hotTTR comments to WANObjectCache
Change-Id: I123e5231206350ee51098fcb6528da126d1a86ce
---
M includes/libs/objectcache/WANObjectCache.php
1 file changed, 13 insertions(+), 8 deletions(-)
git pull ssh://gerrit.wikimedia.org:29418/mediawiki/core
refs/changes/13/386113/1
diff --git a/includes/libs/objectcache/WANObjectCache.php
b/includes/libs/objectcache/WANObjectCache.php
index 15e5759..6c198f2 100644
--- a/includes/libs/objectcache/WANObjectCache.php
+++ b/includes/libs/objectcache/WANObjectCache.php
@@ -686,8 +686,11 @@
* 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 )
*
@@ -839,11 +842,13 @@
* 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 refreshses 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.
@@ -1534,7 +1539,7 @@
}
/**
- * 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
--
To view, visit https://gerrit.wikimedia.org/r/386113
To unsubscribe, visit https://gerrit.wikimedia.org/r/settings
Gerrit-MessageType: newchange
Gerrit-Change-Id: I123e5231206350ee51098fcb6528da126d1a86ce
Gerrit-PatchSet: 1
Gerrit-Project: mediawiki/core
Gerrit-Branch: master
Gerrit-Owner: Aaron Schulz <asch...@wikimedia.org>
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