beezeeking on "[Plugin: W3 Total Cache] Object cache flushes entire memcached host"

hi,seand11
i have a problem needing your help?the plugin "filter-by-comments",i added the code"orderby_last_activity' => 1",but it dosn't work,how can i let it work?

<?php
	global $wp_query, $paged;
	if( get_query_var( 'paged' ) ) {
		$paged = get_query_var( 'paged' );
	}
	elseif( get_query_var( 'page' ) ) {
		$paged = get_query_var( 'page' );
	}
	else {
		$paged = 1;
	}
	$blog_query = new WP_Query( array( 'post_type' => 'post','orderby_last_activity' => 1, 'paged' => $paged ) );
	$temp_query = $wp_query;
	$wp_query = null;
	$wp_query = $blog_query;

	if ( $blog_query->have_posts() ) :

			while ( $blog_query->have_posts() ) : $blog_query->the_post();
				?>