<?php
+/**
+ * When you have many workers (threads/servers) giving service, and a
+ * cached item expensive to produce expires, you may get several workers
+ * doing the job at the same time.
+ * Given enough requests and the item expiring fast (non-cacheable,
+ * lots of edits...) that single work can end up unfairly using most (all)
+ * of the cpu of the pool. This is also known as 'Michael Jackson effect'.
+ * The PoolCounter provides semaphore semantics for restricting the number
+ * of workers that may be concurrently performing such single task.
+ *
+ * By default PoolCounter_Stub is used, which provides no locking. You
+ * can get a useful one in the PoolCounter extension.
+ */
abstract class PoolCounter {
+
+ /* Return codes */
+ const LOCKED = 1; /* Lock acquired */
+ const RELEASED = 2; /* Lock released */
+ const DONE = 3; /* Another one did the work for you */
+
+ const ERROR = -1; /* Indeterminate error */
+ const NOT_LOCKED = -2; /* Called release() with no lock held */
+ const QUEUE_FULL = -3; /* There are already maxqueue workers on this lock */
+ const TIMEOUT = -4; /* Timeout exceeded */
+ const LOCK_HELD = -5; /* Cannot acquire another lock while you have one lock held */
+
+ /**
+ * I want to do this task and I need to do it myself.
+ *
+ * @return Locked/Error
+ */
+ abstract function acquireForMe();
+
+ /**
+ * I want to do this task, but if anyone else does it
+ * instead, it's also fine for me. I will read its cached data.
+ *
+ * @return Locked/Done/Error
+ */
+ abstract function acquireForAnyone();
+
+ /**
+ * I have successfully finished my task.
+ * Lets another one grab the lock, and returns the workers
+ * waiting on acquireForAnyone()
+ *
+ * @return Released/NotLocked/Error
+ */
+ abstract function release();
+
+ /**
+ * $key: All workers with the same key share the lock.
+ * $workers: It wouldn't be a good idea to have more than this number of
+ * workers doing the task simultaneously.
+ * $maxqueue: If this number of workers are already working/waiting,
+ * fail instead of wait.
+ * $timeout: Maximum time in seconds to wait for the lock.
+ */
+ protected $key, $workers, $maxqueue, $timeout;
+
+ /**
+ * Create a Pool counter. This should only be called from the PoolWorks.
+ */
public static function factory( $type, $key ) {
global $wgPoolCounterConf;
if ( !isset( $wgPoolCounterConf[$type] ) ) {
}
$conf = $wgPoolCounterConf[$type];
$class = $conf['class'];
+
return new $class( $conf, $type, $key );
}
-
- abstract public function acquire();
- abstract public function release();
- abstract public function wait();
-
- public function executeProtected( $mainCallback, $dirtyCallback = false ) {
- $status = $this->acquire();
- if ( !$status->isOK() ) {
- return $status;
- }
- if ( !empty( $status->value['overload'] ) ) {
- # Overloaded. Try a dirty cache entry.
- if ( $dirtyCallback ) {
- if ( call_user_func( $dirtyCallback ) ) {
- $this->release();
- return Status::newGood();
- }
- }
-
- # Wait for a thread
- $status = $this->wait();
- if ( !$status->isOK() ) {
- $this->release();
- return $status;
- }
- }
- # Call the main callback
- call_user_func( $mainCallback );
- return $this->release();
+
+ protected function __construct( $conf, $type, $key ) {
+ $this->key = $key;
+ $this->workers = $conf['workers'];
+ $this->maxqueue = $conf['maxqueue'];
+ $this->timeout = $conf['timeout'];
}
}
class PoolCounter_Stub extends PoolCounter {
- public function acquire() {
- return Status::newGood();
+ function acquireForMe() {
+ return Status::newGood( PoolCounter::LOCKED );
}
- public function release() {
- return Status::newGood();
+ function acquireForAnyone() {
+ return Status::newGood( PoolCounter::LOCKED );
}
- public function wait() {
- return Status::newGood();
+ function release() {
+ return Status::newGood( PoolCounter::RELEASED );
}
-
- public function executeProtected( $mainCallback, $dirtyCallback = false ) {
- call_user_func( $mainCallback );
- return Status::newGood();
+
+ public function __construct() {
+ /* No parameters needed */
}
}
+/**
+ * Handy class for dealing with PoolCounters using class members instead of callbacks.
+ */
+abstract class PoolCounterWork {
+ protected $cacheable = false; //Does this override getCachedWork() ?
+
+ /**
+ * Actually perform the work, caching it if needed.
+ */
+ abstract function doWork();
+ /**
+ * Retrieve the work from cache
+ * @return mixed work result or false
+ */
+ function getCachedWork() {
+ return false;
+ }
+
+ /**
+ * A work not so good (eg. expired one) but better than an error
+ * message.
+ * @return mixed work result or false
+ */
+ function fallback() {
+ return false;
+ }
+
+ /**
+ * Do something with the error, like showing it to the user.
+ */
+ function error( $status ) {
+ return false;
+ }
+
+ /**
+ * Get the result of the work (whatever it is), or false.
+ */
+ function execute( $skipcache = false ) {
+ if ( $this->cacheable && !$skipcache ) {
+ $status = $this->poolCounter->acquireForAnyone();
+ } else {
+ $status = $this->poolCounter->acquireForMe();
+ }
+
+ if ( $status->isOK() ) {
+ switch ( $status->value ) {
+ case PoolCounter::LOCKED:
+ $result = $this->doWork();
+ $this->poolCounter->release();
+ return $result;
+
+ case PoolCounter::DONE:
+ $result = $this->getCachedWork();
+ if ( $result === false ) {
+ /* That someone else work didn't serve us.
+ * Acquire the lock for me
+ */
+ return $this->execute( true );
+ }
+ return $result;
+
+ case PoolCounter::QUEUE_FULL:
+ case PoolCounter::TIMEOUT:
+ $result = $this->fallback();
+
+ if ( $result !== false ) {
+ return $result;
+ }
+ /* no break */
+
+ /* These two cases should never be hit... */
+ case PoolCounter::ERROR:
+ default:
+ $errors = array( PoolCounter::QUEUE_FULL => 'pool-queuefull', PoolCounter::TIMEOUT => 'pool-timeout' );
+
+ $status = Status::newFatal( isset($errors[$status->value]) ? $errors[$status->value] : 'pool-errorunknown' );
+ /* continue to the error */
+ }
+ }
+ return $this->error( $status );
+ }
+
+ function __construct( $type, $key ) {
+ $this->poolCounter = PoolCounter::factory( $type, $key );
+ }
+}
scripts.mit.edu / autoinstallsdev / mediawiki.git / blobdiff