Memcache几年前我们就开始在Drupal站上启用，对于降低数据库负载的作用很明显，偶尔会出现Memcache自己停了需要重启Memcache服务的情况，但很少很少，几个月也难得一次。
6月底开始申请熊掌号后，重新提交URL、提交MIP版本/AMP版本，带来大量百度爬虫，服务器负载明显增高，另外其它采集者也多起来了，导致我们网站有一阵子总是卡住，甚至随机出现白屏打不开的情况，这种白屏故障会一直持续，直到用drush cc all清理缓存后恢复。
这对网站的访问影响太大了，如果没有及时发现，可能几个小时、半天都打不开，仔细查看RDS的实时查询进程，发现一种block很容易堆积，我们使用专门的SQL语句查询来替换了Drupal自身的block，负载可以明显下降，但白屏情况依然不能杜绝。
再仔细排查，当出现白屏故障的时候，我们用service memcached restart重启memcached服务，网站可以恢复访问，这就说明是memcache的缓存出现了问题，重点来排查memcache的设置。
从Drupal的Memcache模块的README.txt文件阅读配置办法，发现有两个地方我们以前没有留意、没有特别设置：
## LOCKING ## The memcache-lock.inc file included with this module can be used as a drop-in replacement for the database-mediated locking mechanism provided by Drupal core. To enable, define the following in your settings.php: $conf['lock_inc'] = 'sites/all/modules/memcache/memcache-lock.inc'; Locks are written in the 'semaphore' table, which will map to the 'default' memcache cluster unless you explicitly configure a 'semaphore' cluster. ## STAMPEDE PROTECTION ## Memcache includes stampede protection for rebuilding expired and invalid cache items. To enable stampede protection, define the following in settings.php: $conf['memcache_stampede_protection'] = TRUE; To avoid lock stampedes, it is important that you enable the memcache lock implementation when enabling stampede protection -- enabling stampede protection without enabling the Memcache lock implementation can cause worse performance and can result in dropped locks due to key-length truncation. Memcache stampede protection is primarily designed to benefit the following caching pattern: a miss on a cache_get() for a specific cid is immediately followed by a cache_set() for that cid. Of course, this is not the only caching pattern used in Drupal, so stampede protection can be selectively disabled for optimal performance. For example, a cache miss in Drupal core's module_implements() won't execute a cache_set until drupal_page_footer() calls module_implements_write_cache() which can occur much later in page generation. To avoid long hanging locks, stampede protection should be disabled for these delayed caching patterns.
在流量很高的情况下，没有设置这个锁定和踩踏保护机制，就可能出现缓存出错的情况。在原来的settings.php文件中添加以下两行：
$conf['lock_inc'] = 'sites/all/modules/memcache/memcache-lock.inc'; $conf['memcache_stampede_protection'] = TRUE;
添加后就再也没有出现过那种白屏错误的情况。特此记录下来给其他可能遇到类似问题的朋友参考。
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