Enable query classifier cache by default

Enabling it will give a performance improvement and it offloads work from
the CPU. This leads to better and more complete utilization of system
resources.

Documentation/Getting-Started/Configuration-Guide.md

@@ -552,26 +552,29 @@ _qc_sqlite_ - supports the following arguments:
 
 #### `query_classifier_cache_size`
 
-If present, enables and specifies the maximum size of the query classifier cache.
+Specifies the maximum size of the query classifier cache. The default limit is
+40% of total system memory.
 
-If the query classifier cache has been enabled, MaxScale will, after a statement
-has been parsed, store the classification result using the canonicalized version
-of the statement as the key.
+When the query classifier cache has been enabled, MaxScale will, after a
+statement has been parsed, store the classification result using the
+canonicalized version of the statement as the key.
 
-When the classification result for a statement is needed, MaxScale will first
-canonicalize the statement and check whether the result can be found in the cache.
-If it can, the statement will not be parsed at all but the cached result is used.
+If the classification result for a statement is needed, MaxScale will first
+canonicalize the statement and check whether the result can be found in the
+cache.  If it can, the statement will not be parsed at all but the cached result
+is used.
+
+The configuration parameter takes one integer that specifies the maximum size of
+the cache. The size of the cache can be specifed as explained [here](#sizes).
 
-The configuration parameter takes one integer that specifies the maximum size
-of the cache. The size of the cache can be specifed as explained [here](#sizes).
 ```
 # 1MB query classifier cache
-query_classifier_cache=1MB
+query_classifier_cache_size=1MB
 ```
 
 Note that MaxScale uses a separate cache for each worker thread. So, the total
-amount of memory used at most by the cache is obtained by multiplying the
-size specified here, with the number the worker threads used.
+amount of memory used at most by the cache is obtained by multiplying the size
+specified here, with the value of `threads`.
 
 ##### `log_unrecognized_statements`
 

include/maxscale/utils.h

@@ -154,8 +154,20 @@ bool clean_up_pathname(char *path);
 
 bool mxs_mkdir_all(const char *path, int mask);
 
+/**
+ * Return the number of processors
+ *
+ * @return Number of processors or 1 if the information is not available
+ */
 long get_processor_count();
 
+/**
+ * Return total system memory
+ *
+ * @return Total memory in bytes or 0 if the information is not available
+ */
+int64_t get_total_memory();
+
 /**
  * Store a 4 byte integer
  *

server/core/config.cc

@@ -1123,6 +1123,22 @@ bool config_load_global(const char *filename)
     {
         log_config_error(filename, rval);
     }
+    else
+    {
+        // Do some post-processing for auto-sized default variables
+        if (gateway.qc_cache_properties.max_size == -1)
+        {
+            int64_t mem_per_thr = get_total_memory() * 0.4 / gateway.n_threads;
+            mxb_assert(mem_per_thr >= 0);
+            gateway.qc_cache_properties.max_size = mem_per_thr;
+
+            if (mem_per_thr == 0)
+            {
+                MXS_WARNING("Could not auto-detect total system memory for the query classifier "
+                            "cache. Manually define `%s` to enable it.", CN_QUERY_CLASSIFIER_CACHE_SIZE);
+            }
+        }
+    }
 
     return rval == 0;
 }
@@ -2608,6 +2624,9 @@ void config_set_global_defaults()
     gateway.passive = false;
     gateway.promoted_at = 0;
 
+    // Note: This is not a valid cache value: it is used to detect that the default value is used
+    gateway.qc_cache_properties.max_size = -1;
+
     gateway.thread_stack_size = 0;
     gateway.writeq_high_water = 0;
     gateway.writeq_low_water = 0;

server/core/utils.cc

@@ -1119,11 +1119,6 @@ int open_unix_socket(enum mxs_socket_type type, struct sockaddr_un *addr, const
     return fd;
 }
 
-/**
- * Return the number of processors available.
- * @return Number of processors or 1 if the required definition of _SC_NPROCESSORS_CONF
- * is not found
- */
 long get_processor_count()
 {
     long processors = 1;
@@ -1139,6 +1134,24 @@ long get_processor_count()
     return processors;
 }
 
+int64_t get_total_memory()
+{
+    int64_t pagesize = 0;
+    int64_t num_pages = 0;
+#if defined _SC_PAGESIZE && defined _SC_PHYS_PAGES
+    if ((pagesize = sysconf(_SC_PAGESIZE)) <= 0 || (num_pages = sysconf(_SC_PHYS_PAGES)) <= 0)
+    {
+        MXS_WARNING("Unable to establish total system memory");
+        pagesize = 0;
+        num_pages = 0;
+    }
+#else
+#error _SC_PAGESIZE and _SC_PHYS_PAGES are not defined
+#endif
+    mxb_assert(pagesize * num_pages > 0);
+    return pagesize * num_pages;
+}
+
 namespace maxscale
 {