server: set globalCacheSize honoring system limits for max memory. (#2332)

On unix systems it is possible to set max memory used by
running processes using 'ulimit -m' or 'syscall.RLIMIT_AS'.

A process whence exceeds this limit, kernel would pro-actively
kill such a server with OOM. To avoid this problem of defaulting
our cache size to 8GB we should look for if the current system
limits are lower and set the cache size appropriately.

server-rlimit-nix.go

@@ -30,7 +30,7 @@ func setMaxOpenFiles() error {
 		return err
 	}
 	// Set the current limit to Max, it is usually around 4096.
-	// TO increate this limit further user has to manually edit
+	// TO increase this limit further user has to manually edit
 	// `/etc/security/limits.conf`
 	rLimit.Cur = rLimit.Max
 	err = syscall.Setrlimit(syscall.RLIMIT_NOFILE, &rLimit)
@@ -43,3 +43,34 @@ func setMaxOpenFiles() error {
 	}
 	return nil
 }
+
+// Set max memory used by minio as a process, this value is usually
+// set to 'unlimited' but we need to validate additionally to verify
+// if any hard limit is set by the user, in such a scenario would need
+// to reset the global max cache size to be 80% of the hardlimit set
+// by the user. This is done to honor the system limits and not crash.
+func setMaxMemory() error {
+	var rLimit syscall.Rlimit
+	err := syscall.Getrlimit(syscall.RLIMIT_AS, &rLimit)
+	if err != nil {
+		return err
+	}
+	// Set the current limit to Max, it is default 'unlimited'.
+	// TO decrease this limit further user has to manually edit
+	// `/etc/security/limits.conf`
+	rLimit.Cur = rLimit.Max
+	err = syscall.Setrlimit(syscall.RLIMIT_AS, &rLimit)
+	if err != nil {
+		return err
+	}
+	err = syscall.Getrlimit(syscall.RLIMIT_AS, &rLimit)
+	if err != nil {
+		return err
+	}
+	// Validate if rlimit memory is set to lower
+	// than max cache size. Then we should use such value.
+	if rLimit.Cur < globalMaxCacheSize {
+		globalMaxCacheSize = (80 / 100) * rLimit.Cur
+	}
+	return nil
+}