Add long-term caching

[cmc333333]

Many of our pages take a second or more to load, yet change very rarely. Add an eregs-specific cache which will store these pages for 15 days. Default to a file-system solution so we can easily clear it when new data is present.

On a pre-caching script which loads several hundred pages, this cache caused a 40x speedup.

[cmc333333]

Note that is requires modifying settings (if the base are not used, or are over written).

For development, you probably want:

    'eregs_longterm_cache': {
        'BACKEND': 'django.core.cache.backends.dummy.DummyCache'
    },

and in production:

    'eregs_longterm_cache': {
        'BACKEND': 'django.core.cache.backends.filebased.FileBasedCache',
        'LOCATION': '/tmp/eregs_longterm_cache',
        'TIMEOUT': 60*60*24*15,     # 15 days
        'OPTIONS': {
            'MAX_ENTRIES': 10000,
        },
    },

[cmc333333]

@rosskarchner You may be the best set of eyes to review this

[ascott1]

Ping @rosskarchner. We've certainly gotten feedback that the site can feel slow. I'd love to see this in action.

[rosskarchner]

I'm embarrassed that I just found this, but I think it's gonna do a world of good.

[ascott1]

:+1: no embarrassment necessary. Better late than never!

[rosskarchner]

@ascott1 (and @cmc333333 maybe)-- does this pre-caching script still run? how often?

[cmc333333]

It used to run off of the demo server, which, I believe was re-imaged -- I don't know if it's still running. It was just a bash or python script that hit a whole bunch of urls on a cron. I believe it ran every hour, but I'm not certain. I think I wrote some internal docs about it.

[rosskarchner]

Thanks, CM!

[rosskarchner]

We are now using this in production!

I'm kind of bummed that the page cache isn't shared between servers (so every URL is theoretically generated twice), but we're still thinking through that. Basically: is the increased efficiency of cache usage worth the overhead of a network mount or some other solution?

[ascott1]

Whoa, eRegs screeeeaaaaaammmmssss now. Amazing.

[khandelwal]

Yeah, the speed improvements are AWESOME.

[ascott1]

I'm just clicking ALL THE THINGS.

[khandelwal]

CLICK, CLICK, CLICK.

[cmc333333]

Yay caching. A goal I've had for a long time is to make a static site generator for this, and the content, by definition, almost never changes. The long-term cache is a middle ground -- no significant rework of the underlying system, yet massive performance boost.

@rosskarchner - if you wanted to share across servers, memcached is probably the answer. The only tricky bit would be to verify timeouts. Luckily, you can configure that all without changing the eregs code base :)

[rosskarchner]

Things may have gotten a bit faster (or I'm imagining things), as these servers are now using SSD's

[cmc333333]

@ascott1 I revise:

For development, you probably want:

    'eregs_longterm_cache': {
        'BACKEND': 'django.core.cache.backends.dummy.DummyCache',
        'TIMEOUT': 60*60*24*15,     # 15 days
    },