Cache.expire() should cull items to respect size_limit

[mrclary]

Grant,
I couldn't find a way to contact you regarding a question about DiskCache, except to file an issue.
I do not know how to cull items from the cache if the cache size is larger than the size_limit.

If I fill the cache to something larger than the size_limit, the cache does not report any items as expired, so calling the expire method does nothing. How is the size_limit parameter supposed to work?

[grantjenks]

The size_limit parameter should limit the size of the cache. It's a soft-limit though. How big is the cache getting?

Can you post some code to repro?

[grantjenks]

Also note that if you set the cull_limit to zero then the size_limit is ignored.

[grantjenks]

Another thing: expire() will only evict items that have expired from the cache. It won't evict for size constraints.

Maybe I understand your issue: do you have cull_limit set to zero? and are you calling expire() from a separate process to remove items? but observing that the cache is not obeying the size_limit? I could believe that's happening and that sounds like a bug.

[mrclary]

That's exactly the issue I'm having. I expected expire() to trim the cache to the size_limit, using the eviction policy parameter. I do have the cull_limit set to zero.

What I would like to do is cull items manually, which is why I set cull_limit=0. I don't want items culled unless I call expire() explicitly. When I do cull items (by calling expire()) I'd like items removed according to the eviction policy (e.g. least-recently-stored) and meet both the expire time AND size_limit parameters.

Does a cache item only get flagged as expired if the expire time parameter is met?
How does the cache size "soft-limit" work?

[mrclary]

I don't need a hard size limit. I just want to make sure I understand how it's supposed to work. I couldn't find anything in the documentation specifically addressing how the cache size limit is maintained, so I couldn't be sure if I was expecting the correct behavior.

[grantjenks]

I agree. That's a bug. I expected expire() to remove items to meet the size limit too.

I can best explain the "soft limit" with an example. Suppose the cache was initialized with its defaults: size_limit = 1GB and cull_limit = 10. Then suppose you fill the cache with one million 1KB items. That would put the cache right at the 1GB size limit. If you then store a 1MB item then to meet the size_limit, you need to evict one thousand 1KB items. But with the cull_limit set to 10, that won't happen. At least, not immediately. The cull_limit will limit the number of items removed during culling (which happens when an item is set() in the cache).

Does that make sense?

[grantjenks]

Renamed issue to capture the scenario. You can work-around the issue by doing something like the following:

def cull(cache):
    cache.cull_limit = 10  # Do NOT use reset()
    while cache.volume() > cache.size_limit:
        cache._cull()

[mrclary]

did you mean the following?

def cull(cache):
    cache.cull_limit = 10  # Do NOT use reset()
    now = time.time()
    while cache.volume() > cache.size_limit:
        with cache._transact() as (sql, cleanup):
            cache._cull(now, sql, cleanup)

[grantjenks]

Yep, that looks right. That should work in a separate process. Careful about setting the cull_limit. Probably better to do:

def cull(path):
    with diskcache.Cache(path, timeout=60) as cache:
        cache.cull_limit = 10  # Do NOT use reset()
        now = time.time()
        while cache.volume() > cache.size_limit:
            with cache._transact() as (sql, cleanup):
                cache._cull(now, sql, cleanup)

[grantjenks]

I'm working on this with some other changes. Should be up on PyPI before the New Year.

[grantjenks]

Implementation complete at 434c128 will deploy in v3 to PyPI this week.