github.com/pingcap/badger@v1.5.1-0.20230103063557-828f39b09b6d/cache/README.md (about)

     1  # Ristretto
     2  [![Go Doc](https://img.shields.io/badge/godoc-reference-blue.svg)](http://godoc.org/github.com/dgraph-io/ristretto)
     3  [![Go Report Card](https://img.shields.io/badge/go%20report-A%2B-brightgreen)](https://goreportcard.com/report/github.com/dgraph-io/ristretto)
     4  [![Coverage](https://img.shields.io/badge/coverage-100%25-brightgreen)](https://gocover.io/github.com/dgraph-io/ristretto)
     5  ![Tests](https://github.com/dgraph-io/ristretto/workflows/tests/badge.svg)
     6  
     7  Ristretto is a fast, concurrent cache library built with a focus on performance and correctness.
     8  
     9  The motivation to build Ristretto comes from the need for a contention-free
    10  cache in [Dgraph][].
    11  
    12  [Dgraph]: https://github.com/dgraph-io/dgraph
    13  
    14  ## Features
    15  
    16  * **High Hit Ratios** - with our unique admission/eviction policy pairing, Ristretto's performance is best in class.
    17  	* **Eviction: SampledLFU** - on par with exact LRU and better performance on Search and Database traces.
    18  	* **Admission: TinyLFU** - extra performance with little memory overhead (12 bits per counter).
    19  * **Fast Throughput** - we use a variety of techniques for managing contention and the result is excellent throughput.
    20  * **Cost-Based Eviction** - any large new item deemed valuable can evict multiple smaller items (cost could be anything).
    21  * **Fully Concurrent** - you can use as many goroutines as you want with little throughput degradation. 
    22  * **Metrics** - optional performance metrics for throughput, hit ratios, and other stats.
    23  * **Simple API** - just figure out your ideal `Config` values and you're off and running.
    24  
    25  ## Status
    26  
    27  Ristretto is usable but still under active development. We expect it to be production ready in the near future.
    28  
    29  ## Table of Contents
    30  
    31  * [Usage](#Usage)
    32  	* [Example](#Example)
    33  	* [Config](#Config)
    34  		* [NumCounters](#Config)
    35  		* [MaxCost](#Config)
    36  		* [BufferItems](#Config)
    37  		* [Metrics](#Config)
    38  		* [OnEvict](#Config)
    39  		* [KeyToHash](#Config)
    40          * [Cost](#Config)
    41  * [Benchmarks](#Benchmarks)
    42  	* [Hit Ratios](#Hit-Ratios)
    43  		* [Search](#Search)
    44  		* [Database](#Database)
    45  		* [Looping](#Looping)
    46  		* [CODASYL](#CODASYL)
    47  	* [Throughput](#Throughput)
    48  		* [Mixed](#Mixed)
    49  		* [Read](#Read)
    50  		* [Write](#Write)
    51  * [FAQ](#FAQ)
    52  
    53  ## Usage
    54  
    55  ### Example
    56  
    57  ```go
    58  func main() {
    59  	cache, err := ristretto.NewCache(&ristretto.Config{
    60  		NumCounters: 1e7,     // number of keys to track frequency of (10M).
    61  		MaxCost:     1 << 30, // maximum cost of cache (1GB).
    62  		BufferItems: 64,      // number of keys per Get buffer.
    63  	})
    64  	if err != nil {
    65  		panic(err)
    66  	}
    67  
    68  	// set a value with a cost of 1
    69  	cache.Set("key", "value", 1)
    70  	
    71  	// wait for value to pass through buffers
    72  	time.Sleep(10 * time.Millisecond)
    73  
    74  	value, found := cache.Get("key")
    75  	if !found {
    76  		panic("missing value")
    77  	}
    78  	fmt.Println(value)
    79  	cache.Del("key")
    80  }
    81  ```
    82  
    83  ### Config
    84  
    85  The `Config` struct is passed to `NewCache` when creating Ristretto instances (see the example above). 
    86  
    87  **NumCounters** `int64`
    88  
    89  NumCounters is the number of 4-bit access counters to keep for admission and eviction. We've seen good performance in setting this to 10x the number of items you expect to keep in the cache when full. 
    90  
    91  For example, if you expect each item to have a cost of 1 and MaxCost is 100, set NumCounters to 1,000. Or, if you use variable cost values but expect the cache to hold around 10,000 items when full, set NumCounters to 100,000. The important thing is the *number of unique items* in the full cache, not necessarily the MaxCost value. 
    92  
    93  **MaxCost** `int64`
    94  
    95  MaxCost is how eviction decisions are made. For example, if MaxCost is 100 and a new item with a cost of 1 increases total cache cost to 101, 1 item will be evicted. 
    96  
    97  MaxCost can also be used to denote the max size in bytes. For example, if MaxCost is 1,000,000 (1MB) and the cache is full with 1,000 1KB items, a new item (that's accepted) would cause 5 1KB items to be evicted. 
    98  
    99  MaxCost could be anything as long as it matches how you're using the cost values when calling Set. 
   100  
   101  **BufferItems** `int64`
   102  
   103  BufferItems is the size of the Get buffers. The best value we've found for this is 64. 
   104  
   105  If for some reason you see Get performance decreasing with lots of contention (you shouldn't), try increasing this value in increments of 64. This is a fine-tuning mechanism and you probably won't have to touch this.
   106  
   107  **Metrics** `bool`
   108  
   109  Metrics is true when you want real-time logging of a variety of stats. The reason this is a Config flag is because there's a 10% throughput performance overhead. 
   110  
   111  **OnEvict** `func(hashes [2]uint64, value interface{}, cost int64)`
   112  
   113  OnEvict is called for every eviction.
   114  
   115  **KeyToHash** `func(key interface{}) [2]uint64`
   116  
   117  KeyToHash is the hashing algorithm used for every key. If this is nil, Ristretto has a variety of [defaults depending on the underlying interface type](https://github.com/dgraph-io/ristretto/blob/master/z/z.go#L19-L41).
   118  
   119  Note that if you want 128bit hashes you should use the full `[2]uint64`,
   120  otherwise just fill the `uint64` at the `0` position and it will behave like
   121  any 64bit hash.
   122  
   123  **Cost** `func(value interface{}) int64`
   124  
   125  Cost is an optional function you can pass to the Config in order to evaluate
   126  item cost at runtime, and only for the Set calls that aren't dropped (this is
   127  useful if calculating item cost is particularly expensive and you don't want to
   128  waste time on items that will be dropped anyways).
   129  
   130  To signal to Ristretto that you'd like to use this Cost function:
   131  
   132  1. Set the Cost field to a non-nil function.
   133  2. When calling Set for new items or item updates, use a `cost` of 0.
   134  
   135  ## Benchmarks
   136  
   137  The benchmarks can be found in https://github.com/dgraph-io/benchmarks/tree/master/cachebench/ristretto.
   138  
   139  ### Hit Ratios
   140  
   141  #### Search
   142  
   143  This trace is described as "disk read accesses initiated by a large commercial
   144  search engine in response to various web search requests."
   145  
   146  <p align="center">
   147  	<img src="https://raw.githubusercontent.com/karlmcguire/karlmcguire.com/master/docs/Hit%20Ratios%20-%20Search%20(ARC-S3).svg?sanitize=true">
   148  </p>
   149  
   150  #### Database
   151  
   152  This trace is described as "a database server running at a commercial site
   153  running an ERP application on top of a commercial database."
   154  
   155  <p align="center">
   156  	<img src="https://raw.githubusercontent.com/karlmcguire/karlmcguire.com/master/docs/Hit%20Ratios%20-%20Database%20(ARC-DS1).svg?sanitize=true">
   157  </p>
   158  
   159  #### Looping
   160  
   161  This trace demonstrates a looping access pattern.
   162  
   163  <p align="center">
   164  	<img src="https://raw.githubusercontent.com/karlmcguire/karlmcguire.com/master/docs/Hit%20Ratios%20-%20Glimpse%20(LIRS-GLI).svg?sanitize=true">
   165  </p>
   166  
   167  #### CODASYL
   168  
   169  This trace is described as "references to a CODASYL database for a one hour
   170  period."
   171  
   172  <p align="center">
   173  	<img src="https://raw.githubusercontent.com/karlmcguire/karlmcguire.com/master/docs/Hit%20Ratios%20-%20CODASYL%20(ARC-OLTP).svg?sanitize=true">
   174  </p>
   175  
   176  ### Throughput
   177  
   178  All throughput benchmarks were ran on an Intel Core i7-8700K (3.7GHz) with 16gb
   179  of RAM.
   180  
   181  #### Mixed
   182  
   183  <p align="center">
   184  	<img src="https://raw.githubusercontent.com/karlmcguire/karlmcguire.com/master/docs/Throughput%20-%20Mixed.svg?sanitize=true">
   185  </p>
   186  
   187  #### Read
   188  
   189  <p align="center">
   190  	<img src="https://raw.githubusercontent.com/karlmcguire/karlmcguire.com/master/docs/Throughput%20-%20Read%20(Zipfian).svg?sanitize=true">
   191  </p>
   192  
   193  #### Write
   194  
   195  <p align="center">
   196  	<img src="https://raw.githubusercontent.com/karlmcguire/karlmcguire.com/master/docs/Throughput%20-%20Write%20(Zipfian).svg?sanitize=true">
   197  </p>
   198  
   199  ## FAQ
   200  
   201  ### How are you achieving this performance? What shortcuts are you taking?
   202  
   203  We go into detail in the [Ristretto blog post](https://blog.dgraph.io/post/introducing-ristretto-high-perf-go-cache/), but in short: our throughput performance can be attributed to a mix of batching and eventual consistency. Our hit ratio performance is mostly due to an excellent [admission policy](https://arxiv.org/abs/1512.00727) and SampledLFU eviction policy.
   204  
   205  As for "shortcuts," the only thing Ristretto does that could be construed as one is dropping some Set calls. That means a Set call for a new item (updates are guaranteed) isn't guaranteed to make it into the cache. The new item could be dropped at two points: when passing through the Set buffer or when passing through the admission policy. However, this doesn't affect hit ratios much at all as we expect the most popular items to be Set multiple times and eventually make it in the cache. 
   206  
   207  ### Is Ristretto distributed?
   208  
   209  No, it's just like any other Go library that you can import into your project and use in a single process.