On 10 Dec 2013, at 17:48, [email protected] wrote: > Looks useful indeed. Thx.
Thanks. > I like the useSemaphore thing. If you need it, it is necessary. Otherwise it slows things down. > Question: what does Neo stands for as prefix? I wonder. Makes me think of The > Matrix and Red Pills. Ah, it’s just the name of a namespace, I usually think of it as ‘New’, but yes there is a Matrix link also ;-) > Phil > > > > > On Tue, Dec 10, 2013 at 4:54 PM, Sven Van Caekenberghe <[email protected]> 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 > > >
