Add S3-FIFO RAM cache eviction algorithm (ram_cache.algorithm = 2)

doc/admin-guide/files/records.yaml.en.rst

@@ -2892,10 +2892,28 @@ RAM Cache
 
 .. ts:cv:: CONFIG proxy.config.cache.ram_cache.algorithm INT 1
 
-   Two distinct RAM caches are supported, the default (1) being the simpler
-   **LRU** (*Least Recently Used*) cache. As an alternative, the **CLFUS**
-   (*Clocked Least Frequently Used by Size*) is also available, by changing this
-   configuration to 0.
+   Three RAM cache eviction algorithms are supported, selected by this value:
+
+   ``1``
+       **LRU** (*Least Recently Used*), the default -- the simplest policy,
+       favoring recency. Pairs with
+       :ts:cv:`proxy.config.cache.ram_cache.use_seen_filter` for scan
+       resistance.
+
+   ``0``
+       **CLFUS** (*Clocked Least Frequently Used by Size*), which balances
+       recency, frequency, and object size. It is the only algorithm that
+       supports in-RAM compression
+       (:ts:cv:`proxy.config.cache.ram_cache.compress`).
+
+   ``2``
+       **S3-FIFO** (*Simple Scalable Static FIFO*): a small admission queue and
+       a main queue (both FIFO), plus a ghost queue of recently evicted keys,
+       which together filter one-hit-wonders. Scan-resistant and inexpensive
+       (no per-hit reordering); strong hit rates on CDN and key-value
+       workloads. Its eviction metadata (including the ghost) is accounted
+       within :ts:cv:`proxy.config.cache.ram_cache.size`. Experimental; it does
+       not use the seen filter or support in-RAM compression.
 
 .. ts:cv:: CONFIG proxy.config.cache.ram_cache.use_seen_filter INT 1
 

doc/admin-guide/storage/index.en.rst

@@ -75,18 +75,22 @@ and reduces load on disks, especially during temporary traffic peaks.
 You can configure the RAM cache size to suit your needs, as described in
 :ref:`changing-the-size-of-the-ram-cache` below.
 
-The RAM cache supports two cache eviction algorithms, a regular *LRU*
-(Least Recently Used) and the more advanced *CLFUS* (Clocked Least
+The RAM cache supports three cache eviction algorithms: a regular *LRU*
+(Least Recently Used); the more advanced *CLFUS* (Clocked Least
 Frequently Used by Size; which balances recentness, frequency, and size
-to maximize hit rate, similar to a most frequently used algorithm).
-The default is to use *LRU*, and this is controlled via
+to maximize hit rate, similar to a most frequently used algorithm); and
+*S3-FIFO* (Simple Scalable Static FIFO), a FIFO-based policy whose small
+admission queue and ghost list filter one-hit-wonders, giving strong hit
+rates on CDN and key-value workloads at low cost. The default is to use
+*LRU*, and this is controlled via
 :ts:cv:`proxy.config.cache.ram_cache.algorithm`.
 
 Both the *LRU* and *CLFUS* RAM caches support a configuration to increase
 scan resistance. In a typical *LRU*, if you request all possible objects in
 sequence, you will effectively churn the cache on every request. The option
 :ts:cv:`proxy.config.cache.ram_cache.use_seen_filter` can be set to add some
-resistance against this problem.
+resistance against this problem. *S3-FIFO* is scan-resistant by design,
+through its admission queue, and does not use the seen filter.
 
 In addition, *CLFUS* also supports compressing in the RAM cache itself.
 This can be useful for content which is not compressed by itself (e.g.

include/iocore/cache/Cache.h

@@ -36,8 +36,9 @@ static constexpr ts::ModuleVersion CACHE_MODULE_VERSION(1, 0);
 
 #define SCAN_KB_PER_SECOND 8192 // 1TB/8MB = 131072 = 36 HOURS to scan a TB
 
-#define RAM_CACHE_ALGORITHM_CLFUS 0
-#define RAM_CACHE_ALGORITHM_LRU   1
+#define RAM_CACHE_ALGORITHM_CLFUS  0
+#define RAM_CACHE_ALGORITHM_LRU    1
+#define RAM_CACHE_ALGORITHM_S3FIFO 2
 
 #define CACHE_COMPRESSION_NONE    0
 #define CACHE_COMPRESSION_FASTLZ  1

include/iocore/eventsystem/Thread.h

@@ -134,6 +134,7 @@ class Thread
   ProxyAllocator openDirEntryAllocator;
   ProxyAllocator ramCacheCLFUSEntryAllocator;
   ProxyAllocator ramCacheLRUEntryAllocator;
+  ProxyAllocator ramCacheS3FIFOEntryAllocator;
   ProxyAllocator evacuationBlockAllocator;
   ProxyAllocator ioDataAllocator;
   ProxyAllocator ioAllocator;

src/iocore/cache/CMakeLists.txt

@@ -33,6 +33,7 @@ add_library(
   PreservationTable.cc
   RamCacheCLFUS.cc
   RamCacheLRU.cc
+  RamCacheS3FIFO.cc
   Store.cc
   Stripe.cc
   StripeSM.cc

src/iocore/cache/CacheProcessor.cc

@@ -1486,6 +1486,10 @@ CacheProcessor::cacheInitialized()
 
     if (gnstripes) {
       // new ram_caches, with algorithm from the config
+      static const char *const ram_cache_algorithm_name[] = {"CLFUS", "LRU", "S3-FIFO"};
+      int const                ram_alg                    = cache_config_ram_cache_algorithm;
+      Dbg(dbg_ctl_cache_init, "ram_cache algorithm = %d (%s)", ram_alg,
+          (ram_alg >= 0 && ram_alg <= RAM_CACHE_ALGORITHM_S3FIFO) ? ram_cache_algorithm_name[ram_alg] : "unknown");
       for (int i = 0; i < gnstripes; i++) {
         switch (cache_config_ram_cache_algorithm) {
         default:
@@ -1495,6 +1499,9 @@ CacheProcessor::cacheInitialized()
         case RAM_CACHE_ALGORITHM_LRU:
           gstripes[i]->ram_cache = new_RamCacheLRU();
           break;
+        case RAM_CACHE_ALGORITHM_S3FIFO:
+          gstripes[i]->ram_cache = new_RamCacheS3FIFO();
+          break;
         }
       }
 

src/iocore/cache/CacheTest.cc

@@ -679,7 +679,8 @@ REGRESSION_TEST(ram_cache)(RegressionTest *t, int level, int *pstatus)
   for (int s = 20; s <= 24; s += 4) {
     int64_t cache_size = 1LL << s;
     *pstatus           = REGRESSION_TEST_PASSED;
-    if (!test_RamCache(t, new_RamCacheLRU(), "LRU", cache_size) || !test_RamCache(t, new_RamCacheCLFUS(), "CLFUS", cache_size)) {
+    if (!test_RamCache(t, new_RamCacheLRU(), "LRU", cache_size) || !test_RamCache(t, new_RamCacheCLFUS(), "CLFUS", cache_size) ||
+        !test_RamCache(t, new_RamCacheS3FIFO(), "S3-FIFO", cache_size)) {
       *pstatus = REGRESSION_TEST_FAILED;
     }
   }

src/iocore/cache/P_RamCache.h

@@ -45,3 +45,4 @@ class RamCache
 
 RamCache *new_RamCacheLRU();
 RamCache *new_RamCacheCLFUS();
+RamCache *new_RamCacheS3FIFO();