[Pharo-dev] [ANN] Neo-Caching

phil at highoctane.be phil at highoctane.be
Tue Dec 10 11:48:30 EST 2013


Looks useful indeed. Thx.

I like the useSemaphore thing.

Question: what does Neo stands for as prefix? I wonder. Makes me think of
The Matrix and Red Pills.

Phil




On Tue, Dec 10, 2013 at 4:54 PM, Sven Van Caekenberghe <sven at stfx.eu> wrote:

> Hi,
>
> Caching is an important technique to trade time (speed of execution) for
> space (memory consumption) in computation. Used correctly, caching can
> significantly improve an application’s performance.
>
> Given the key/value nature of a cache, a Dictionary is often the first
> data structure used for implementing it. This is however not a good idea
> since a simple Dictionary is unbounded by definition and can/will lead to
> explosive cache growth.
>
> Quick and dirty hacks to turn a Dictionary into a least-recently-used
> (LRU) and/or time-to-live (TTL) based limited cache are often incorrect and
> inefficient.
>
> The LRUCache implementation currently in Pharo is too simple and not
> efficient, being O(n) on its main operation.
>
> Pharo deserves and needs a good LRU/TTL cache implementation that
> can/should be used uniformly in various places of the system and in code
> built on top of it. That is why I implemented a proposal.
>
> Neo-Caching is an implementation of both a good LRU cache as well as a TTL
> extension of it. The caches are easy to use, yet have a number of
> interesting features. The implementation is properly O(1) on its main
> operations and is 2 to 3 times faster than LRUCache. The package also
> contains a double linked list implementation. The code comes with a full
> set of unit tests. There are class and method comments and the code is easy
> to read.
>
> The code (which has no dependencies) can be found here
>
>   http://mc.stfx.eu/Neo
>   http://www.smalltalkhub.com/mc/SvenVanCaekenberghe/Neo/main
>
> There is only one package to load. The code has been written in Pharo 3.0
> but it should run on older versions as well.
>
>
> Code
>
>
> Let’s create a 16K ordered collection of keys that have some repetitions
> in them with a reasonable distribution:
>
> data := Array streamContents: [ :out |
>   1 to: 4096 do: [ :each |
>     each primeFactorsOn: out.
>     out nextPut: each ] ].
> data := data collect: [ :each | each asWords ].
>
> By using #asWords the keys have better hash properties. Now, we put all
> this in a cache:
>
> cache := NeoLRUCache new.
> cache maximumWeight: 512.
> cache factory: [ :key | key ].
> data do: [ :each | cache at: each ].
> cache.
>
> ==> a NeoLRUCache(#512 512/512 [ 1 ] [ :key | key ] 72%)
>
> We had a 72% hit ratio, which is good. The cache is of course full,
> 512/512 with 512 entries (not necessarily the same thing, see further).
>
> We can now benchmark this:
>
> [ data do: [ :each | cache at: each ] ] bench.
>
> ==> '35.8 per second.’
>
> And compare it to LRUCache:
>
> cache := LRUCache size: 512 factory: [ :key | key ].
>
> [ data do: [ :each | cache at: each ] ] bench.
>
> ==> '12.6 per second.’
>
>
> Features
>
>
> Instead of just counting the number of entries, which is the default
> behaviour, the concept of weight is used to determine when a cache is full.
> For each cached value, a block or selector is used to compute its weight.
> When adding a new entry causes the weight to exceed a maximum, eviction of
> the least recently used item(s) takes place, until the weight is again
> below the maximum.
>
> cache
>   computeWeight: #sizeInMemory;
>   maximumWeight: 16*1024.
>
> Will keep the cache below 16Kb in real memory usage. This can be very
> useful when caching various sized images or MC packages, for example.
>
> By default, no concurrent access protection takes place, but optionally a
> semaphore for mutual exclusion can be used. This slows down access.
>
> cache useSemaphore.
>
> NeoTTLCache extends NeoLRUCache by maintaining a timestamp for each cached
> value. Upon cache hit, there is a check to see if this timestamp is not
> older than the allowed time to live. If so, the value is stale and will be
> recomputed.
>
> (cache := NeoTTLCache new)
>   timeToLive: 10 minutes;
>   factory: [ :key | ZnEasy get: key ];
>   maximumWeight: 32*1024;
>   computeWeight: #contentLength.
>
> cache at: ‘http://zn.stfx.eu/zn/numbers.txt'.
>
> ==> a ZnResponse(200 OK text/plain;charset=utf-8 71B)
>
> cache
>
> ==> a NeoTTLCache(#1 71/32768 #contentLength [ :key | ZnEasy get: key ] 0%
> 0:00:10:00)
>
> Would be a simple HTTP cache, keeping resolved URLs in memory for 10
> minutes maximum before refreshing them. It uses #contentLength on
> ZnResponse to compute the weight. You can see from the print string that
> there is now 1 entry, while the weight is 17 bytes out of 32768.
>
>
> These are the main points, please have a look at the code. Feedback is
> welcome.
>
> Regards,
>
> Sven
>
>
>
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