Как препятствовать тому, чтобы медленный отвечающий Tomcat заставил Apache замедлиться для ответа?

if for whatever reason tomcat does not handle your ajax requests fast, this reduces the number of requests that your apache can handle. Tomcat is configured to handle 400 requests in parallel, and there is also a default acceptCount of 100. So your tomcat is able to eat up 500 requests - at least: jvm and platform dependant there may even more connection request queued.

worker.worker1.reply_timeout=120000
worker.worker1.socket_timeout=150000

..tells mod_jk to wait about 1.7 days (socket_timeout is in seconds) for socket operations and 2 minutes for single networks packets from tomcat. You should adjust these values, to let mod_jk return an error as early as possible if tomcat is slow.

Let's assume your ajax requests are typically processed within a second with outliers up to two seconds. After beeing processed, the response is sent back at once. Then one may set worker.worker1.reply_timeout=2500, just half a second more. socket_timeout may even be omitted, as it is just a rough value. socket_connect_timeout, that defines how long it may take to connect from apache/mod_jk to tomcat should be added to worker.properties and set to a very low value, e.g. 100. as both sit on the same server. See The Apache Tomcat Connector -Reference for more details.

Every request, that goes from apache to tomcat counts for what you configured with MaxClientsin httpd.conf. The more requests are stuck in tomcat the less may be processed by apache for static content. If you shutdown tomcat in that situation, static content is delivered fast again, as it frees up resources for request processing in mod_jk and apache.

You have configured prefork.cand worker.c in httpd.conf at the same time. I guess prefork.c is the active, as MaxClients is set to 512 and this would match your observations and my interpretation.. ;-)

Telling mod_jk to give up earlier on long running requests to tomcat might help a lot, but you should also think about adjusting the number of client requests handled by apache (MaxClients) and the number of requests that tomcat processes () in parallel. These numbers have to be balanced to what happens during normal operations. Some tracing of page loads may be helpful to see in what proportion these values should be. The absolute value depends on your servers specs, network situation, number of clients etc.