Solving performance problems with Memcache Mutex extension for eZ Publish - Part II: Technical details

eZMutex is used by standard file handler (eZFSFileHandler). In some future post we will try also to compare this solution with a next generation file handler (eZFS2FileHanlder) included as optional module in current stable eZ Publish version.

Technical details

Default eZMutex has 3 main functions: test(), lock() and unclock(). Function test() returns bool value if the mutex exist or not. In the file based version it does 1 or 2 flock() calls:

if ( flock( $fp, LOCK_EX | LOCK_NB ) ) {
    flock( $fp, LOCK_UN );
    return false;
}
return true;

In the memcache version we only call 1 memcache action:

return memcache_get($this->mc,$this->KeyName);

Function lock() makes an exclusive lock on mutex resource. In the file based version it does 1 flock() call, 1 file_exists() call, 2 create_file() calls and 2 rename_file() calls:

if ( flock( $fp, LOCK_EX ) ) {
    $this->clearMeta();
    $this->setMeta( 'timestamp', time() );
    return true;
}
return false;

In the memcache version there, all in all, 4 memcache calls:

if ( memcache_add($this->mc,$this->KeyName,"1",false, $time) ) {
  $this->clearMeta();
  $this->setMeta( 'timestamp', time() );
  return true;
}
return false;

Function unlock() releases lock on mutex resource. In the file based version functions does 1 fclose() and 2 file_delete() calls.

if ( $fp = $this->fp() ) {
  fclose( $fp );
  @unlink( $this->MetaFileName );
  @unlink( $this->FileName );
  $GLOBALS['eZMutex_FP_' . $this->FileName] = false;
}
return false;

In there memcache version there are 2 memcache calls:

memcache_delete($this->mc,$this->MetaKeyName, 0);
memcache_delete($this->mc,$this->KeyName, 0);
return true;

To summarize the comparison on a test-lock-unlock cycle: we replaced 10-11 file system interactions with 7 memcache interactions and in practice that gives huge performance gains.