reintroduce check for vm.overcommit_memory setting (#8603)

CHANGELOG

@@ -1,6 +1,10 @@
 devel
 -----
 
+* show startup warning in case kernel setting `vm.overcommit_memory` is set  
+  to a value of 2 and the jemalloc memory allocator is in use. This combination
+  does not play well together.
+
 * added AQL functions CRC32 and FNV64 for hashing data
 
 * renamed attribute key `openssl-version` in server/client tool version

Documentation/Books/Manual/ReleaseNotes/UpgradingChanges35.md

@@ -110,3 +110,10 @@ output was renamed to `openssl-version-compile-time`.
 This change affects the output produced when starting one of the ArangoDB
 executables (e.g. arangod, arangosh) with the `--version` command. It also 
 changes the attribute name in the detailed response of the `/_api/version` REST API.
+
+### Overcommit settings
+
+On Linux, ArangoDB will now show a startup warning in case the kernel setting 
+`vm.overcommit_memory` is set to a value of 2 and the jemalloc memory allocator 
+is in use. This combination does not play well together, and may lead to the 
+kernel denying arangod's memory allocation requests in more cases than necessary.

lib/ApplicationFeatures/EnvironmentFeature.cpp

@@ -81,6 +81,14 @@ void EnvironmentFeature::prepare() {
     uint64_t v = basics::StringUtils::uint64(value);
 
     if (v == 2) {
+#ifdef ARANGODB_HAVE_JEMALLOC
+      LOG_TOPIC("fadc5", WARN, arangodb::Logger::MEMORY)
+          << "/proc/sys/vm/overcommit_memory is set to a value of 2. this setting has been found to be problematic";
+      LOG_TOPIC("d08d6", WARN, Logger::MEMORY)
+              << "execute 'sudo bash -c \"echo 0 > "
+              << "/proc/sys/vm/overcommit_memory\"'";
+#endif
+
       // from https://www.kernel.org/doc/Documentation/sysctl/vm.txt:
       //
       //   When this flag is 0, the kernel attempts to estimate the amount