Consider using a flat-file cache for the page cache

We've already optimized Backdrop to the point that a database connection is not necessary if you're using an alternative cache-backend like Memcache. Even though the database is unnecessary except for the single query to get the cached page, we still need that database connection just for that single query.

I was experimenting with using a flat-file cache as an alternative, a basic port of https://www.drupal.org/project/filecache. The results were fairly promising running ab -c 100 -n 100 http://backdrop.local/:

Database cache:

Requests per second:    569.45 [#/sec] (mean)
Time per request:       175.608 [ms] (mean)
Time per request:       1.756 [ms] (mean, across all concurrent requests)
Transfer rate:          12078.02 [Kbytes/sec] received

File cache:

Requests per second:    741.53 [#/sec] (mean)
Time per request:       134.857 [ms] (mean)
Time per request:       1.349 [ms] (mean, across all concurrent requests)
Transfer rate:          15727.74 [Kbytes/sec] received

So roughly a 30% performance increase in cached page delivery. The downside is that the disk becomes littered with non-sense cache files. But as far as disk-space goes, this is no different from the page cache, which takes up just as much room, but it's hidden away in the database.

If we can test this approach on various servers and consistently get a similar increase, I think we might consider bundling a flat-file cache and using it by default for the page cache.