Use MADV_WILLNEED when loading TSM files

When the TSM index is large, this hints to the kernel to start
faulting in pages to avoid lots of smaller page faults.

tsdb/engine/tsm1/reader.go

@@ -11,6 +11,7 @@ import (
 	"sort"
 	"sync"
 	"sync/atomic"
+	"syscall"
 
 	"github.com/influxdata/influxdb/pkg/bytesutil"
 )
@@ -1374,6 +1375,10 @@ func (m *mmapAccessor) init() (*indirectIndex, error) {
 		return nil, fmt.Errorf("mmapAccessor: invalid indexStart")
 	}
 
+	// Hint to the kernal that we will be reading the file.  It would be better to hint
+	// that we will be reading the index section, but that doesn't seem to work ATM.
+	_ = madvise(m.b, syscall.MADV_WILLNEED)
+
 	m.index = NewIndirectIndex()
 	if err := m.index.UnmarshalBinary(m.b[indexStart:indexOfsPos]); err != nil {
 		return nil, err