stale cache sort of bug

  • James

    (@harris1)


    10 years ago

    I’m using the APC object-cache drop in and vastly speeds up requests. And this isn’t exactly a bug in it, but I think it’s the best place to fix it.

    The thing is wp_cache_set. It calls apc_store and that fails if APC is doing an expunge and probably more reasons. But no call to wp_cache_set actually handles that failure. Then the next wp_cache_get returns stale data.

    I patched object-cache.php to log when apc_store fails and tries to delete the stale entry:

    $result = apc_store( $key, $store_data, $expire );
	if ($result === false) {
            apc_fetch($key, $success);
            if ($success) {
                if (apc_delete( $key )) {
                    trigger_error("apc_store($key) failed, deleted old entry");
                } else {
	            trigger_error("apc_store($key) failed, apc_delete also failed. Corruption!");
	        }
            } else {
		trigger_error("apc_store($key) failed, would have been new entry");
	    }
	}
        return $result;

    My logs have several “apc_store($key) failed, deleted old entry” messages.

    Does this make sense? Thanks!

    https://wordpress.org/plugins/apc/

Viewing 4 replies - 1 through 4 (of 4 total)
  • Sam Brodie

    (@sammybeats)

    9 years, 11 months ago

    I’m having the same problem. Have you figured out a fix for it yet?

    Thread Starter James

    (@harris1)

    9 years, 11 months ago

    Kinda/sort of. I replaced the $result = apc_store( $key, $store_data, $expire ); line with the block of code above. The apc_delete call in the snippit clears the stale data, so WP will do a fresh query next request.

    But upgrading and tuning APC is important.

    slam_defense must be disabled since WP updates the same keys often. But in APC 3.1.3pl1 slam_defense cannot be disabled (check the source code, the ini setting is never used).

    APC 3.1.7 introduces read-write locks. Because how can shared memory work with out locks?

    APCu gives user data it’s own memory pool. If I understand correctly, with APC before “u”, the whole cache can be filled of PHP byte code and then every apc_store will fail.

    And APCu’s apc_store still fails if another process is doing an expunge. And we’re back at the workaround.

    Sam Brodie

    (@sammybeats)

    9 years, 11 months ago

    Thanks for the reply, Harris. To use APCu, do you have to uninstall APC first? Or will installing APCu modify your APC installation?

    Thanks again.

    Thread Starter James

    (@harris1)

    9 years, 11 months ago

    I haven’t tried it yet, but my plan is to uninstall APC and install ZendOptimizerPlus and APCu instead.

Viewing 4 replies - 1 through 4 (of 4 total)
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