Fix incorrect Mem.Used calculation (#82)

CHANGELOG.md

@@ -12,6 +12,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Changed
 - OpenBSD returns `ErrNotImplemented` for `ProcTime.Get` instead of `nil`. #83
+- Fixed incorrect `Mem.Used` calculation under linux. #82
 - Fixed `ProcState` on Linux and FreeBSD when process names contain parentheses. #81
 
 ### Deprecated

sigar_linux_common.go

@@ -70,7 +70,7 @@ func (self *Mem) Get() error {
 		self.ActualFree = self.Free + buffers + cached
 	}
 
-	self.Used = self.Total - self.ActualFree
+	self.Used = self.Total - self.Free
 	self.ActualUsed = self.Total - self.ActualFree
 
 	return nil

sigar_linux_test.go

@@ -221,6 +221,9 @@ DirectMap2M:      333824 kB
 	if assert.NoError(t, mem.Get()) {
 		assert.Equal(t, uint64(374256*1024), mem.Total)
 		assert.Equal(t, uint64(274460*1024), mem.Free)
+		assert.Equal(t, uint64(mem.Total-mem.Free), mem.Used)
+		assert.Equal(t, uint64((274460+9764+38648)*1024), mem.ActualFree)
+		assert.Equal(t, uint64(mem.Total-mem.ActualFree), mem.ActualUsed)
 	}
 
 	swap := sigar.Swap{}
@@ -293,6 +296,8 @@ DirectMap2M:      460800 kB
 		assert.Equal(t, uint64(500184*1024), mem.Total)
 		assert.Equal(t, uint64(31360*1024), mem.Free)
 		assert.Equal(t, uint64(414168*1024), mem.ActualFree)
+		assert.Equal(t, uint64(mem.Total-mem.Free), mem.Used)
+		assert.Equal(t, uint64(mem.Total-mem.ActualFree), mem.ActualUsed)
 	}
 
 	swap := sigar.Swap{}