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Make WAN cache HOLDOFF_TTL smaller by combining db/snapshot lag

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* In the common ~0 second lag case, transactions up to ~7 seconds
  long will have normal set() behavior (instead of just ~5 seconds).
* Like wise with ~0 second transactions tolerating ~7 seconds of
  lag (instead of just ~5).
* The lower hold-off time lets caching resume 3 seconds sooner.

Change-Id: I21e2a0a6915492cec422b6a6af5541937745c15b
This commit is contained in:
Aaron Schulz 2015-10-25 13:42:21 -07:00
parent 8f15bc8ec9
commit 05a3ee2468
2 changed files with 49 additions and 16 deletions
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37
includes/libs/objectcache/WANObjectCache.php
View file

@ -81,12 +81,10 @@ class WANObjectCache implements LoggerAwareInterface {
/** Max time expected to pass between delete() and DB commit finishing */
const MAX_COMMIT_DELAY = 3;
/** Max replication lag before applying TTL_LAGGED to set() */
const MAX_REPLICA_LAG = 5;
/** Max time since snapshot transaction start to avoid no-op of set() */
const MAX_SNAPSHOT_LAG = 5;
/** Max replication+snapshot lag before applying TTL_LAGGED or disallowing set() */
const MAX_READ_LAG = 7;
/** Seconds to tombstone keys on delete() */
const HOLDOFF_TTL = 14; // MAX_COMMIT_DELAY + MAX_REPLICA_LAG + MAX_SNAPSHOT_LAG + 1
const HOLDOFF_TTL = 11; // MAX_COMMIT_DELAY + MAX_READ_LAG + 1
/** Seconds to keep dependency purge keys around */
const CHECK_KEY_TTL = 31536000; // 1 year
@ -327,27 +325,34 @@ class WANObjectCache implements LoggerAwareInterface {
$lockTSE = isset( $opts['lockTSE'] ) ? $opts['lockTSE'] : self::TSE_NONE;
$age = isset( $opts['since'] ) ? max( 0, microtime( true ) - $opts['since'] ) : 0;
$lag = isset( $opts['lag'] ) ? $opts['lag'] : 0;
// Disallow set() if the source data is uncommitted as it might get rolled back
if ( !empty( $opts['pending'] ) ) {
$this->logger->info( "Rejected set() for $key due to pending writes." );
return true; // no-op the write for being unsafe
}
if ( $lag > self::MAX_REPLICA_LAG ) {
// Too much lag detected; lower TTL so it converges faster
$ttl = $ttl ? min( $ttl, self::TTL_LAGGED ) : self::TTL_LAGGED;
$this->logger->warning( "Lowered set() TTL for $key due to replication lag." );
}
if ( $age > self::MAX_SNAPSHOT_LAG ) {
// Check if there's a risk of writing stale data after the purge tombstone expired
if ( ( $lag + $age ) > self::MAX_READ_LAG ) {
if ( $lockTSE >= 0 ) {
// Focus on avoiding stampedes; stash the value with a low TTL
$tempTTL = max( 1, (int)$lockTSE ); // set() expects seconds
$this->cache->set( self::STASH_KEY_PREFIX . $key, $value, $tempTTL );
}
$this->logger->warning( "Rejected set() for $key due to snapshot lag." );
// Case A: any long-running transaction; ignore this set()
if ( $age > self::MAX_READ_LAG ) {
$this->logger->warning( "Rejected set() for $key due to snapshot lag." );
return true; // no-op the write for being unsafe
return true; // no-op the write for being unsafe
// Case B: replication lag is high; lower TTL instead of ignoring all set()s
} elseif ( $lag > self::MAX_READ_LAG ) {
$ttl = $ttl ? min( $ttl, self::TTL_LAGGED ) : self::TTL_LAGGED;
$this->logger->warning( "Lowered set() TTL for $key due to replication lag." );
// Case C: medium length request during medium lag; ignore this set()
} else {
$this->logger->warning( "Rejected set() for $key due to high read lag." );
return true; // no-op the write for being unsafe
}
}
$wrapped = $this->wrap( $value, $ttl );

28
tests/phpunit/includes/libs/objectcache/WANObjectCacheTest.php
View file

@ -313,4 +313,32 @@ class WANObjectCacheTest extends MediaWikiTestCase {
$t6 = $this->cache->getCheckKeyTime( $key );
$this->assertEquals( $t5, $t6, 'Check key time did not change' );
}
public function testSetWithLag() {
$value = 1;
$key = wfRandomString();
$opts = array( 'lag' => 300, 'since' => microtime( true ) );
$this->cache->set( $key, $value, 30, $opts );
$this->assertEquals( $value, $this->cache->get( $key ), "Rep-lagged value written." );
$key = wfRandomString();
$opts = array( 'lag' => 0, 'since' => microtime( true ) - 300 );
$this->cache->set( $key, $value, 30, $opts );
$this->assertEquals( false, $this->cache->get( $key ), "Trx-lagged value not written." );
$key = wfRandomString();
$opts = array( 'lag' => 5, 'since' => microtime( true ) - 5 );
$this->cache->set( $key, $value, 30, $opts );
$this->assertEquals( false, $this->cache->get( $key ), "Lagged value not written." );
}
public function testWritePending() {
$value = 1;
$key = wfRandomString();
$opts = array( 'pending' => true );
$this->cache->set( $key, $value, 30, $opts );
$this->assertEquals( false, $this->cache->get( $key ), "Pending value not written." );
}
}