How to debug an empty cache_page table I have a bunch of sites built off the same profile, and recently noticed the pages aren't being cached. I spent some time doing the usual: looking for drupal_set_message calls, disabling all the modules that might disable page cache, as defined here. I've also dug into drupal_page_get_cache. This lead me to the database, where it turns out the cache_page table is empty. 

This is clearly the source of the issue, I'm just not sure how/what is preventing the page cache. What I'd like to do is to figure out where page cache is writing to the database, and do a debug_backtrace, or just dpm from that point backward. 

A) Is there a point in drupal core where cache_page is written to?
B) Any other ideas for debuggings this? (please don't say disable all modules, then re-enable, we have upwards of 200 modules in place with a dozen custom module with interdependencies)

After posting this I opened my IDE and realized I had found the DrupalDatabaseCache-&gt;set method, and that the cache_page $cid is never called into that function.
Whether or not a page is cacheable is determined by drupal_page_is_cacheable().

Set a breakpoint there and dig into call stack / step through the conditions to determine the reason why nothing is saved to the cache_page bin.

Also take a look at drupal_page_footer() which calls into drupal_page_set_cache() in order to store the built page in the page cache.