10990: Adjust readahead to work better on small segments. Improve comments.

diff --git a/sdk/go/keepclient/collectionreader.go b/sdk/go/keepclient/collectionreader.go
index d2a8260b88c6c6b5a535c6f62117b88bbe4d6287..61fabefacdd5ee937faab81c792370bd8af2c675 100644 (file)
--- a/sdk/go/keepclient/collectionreader.go
+++ b/sdk/go/keepclient/collectionreader.go
@@ -72,6 +72,7 @@ type file struct {
        readaheadDone bool
 }
 
+// Close implements io.Closer.
 func (f *file) Close() error {
        f.kc = nil
        f.segments = nil
@@ -79,6 +80,7 @@ func (f *file) Close() error {
        return nil
 }
 
+// Read implements io.Reader.
 func (f *file) Read(buf []byte) (int, error) {
        if f.seg == nil || f.offset < f.segStart || f.offset >= f.segStart+int64(f.seg.Len) {
                // f.seg does not cover the current read offset
@@ -113,16 +115,19 @@ func (f *file) Read(buf []byte) (int, error) {
                }
                f.segData = data[f.seg.Offset : f.seg.Offset+f.seg.Len]
        }
+       // dataOff and dataLen denote a portion of f.segData
+       // corresponding to a portion of the file at f.offset.
        dataOff := int(f.offset - f.segStart)
        dataLen := f.seg.Len - dataOff
 
-       if !f.readaheadDone && len(f.segNext) > 0 && f.offset >= 1048576 && dataOff+dataLen >= 1048576 {
+       if !f.readaheadDone && len(f.segNext) > 0 && f.offset >= 1048576 && dataOff+dataLen > len(f.segData)/16 {
                // If we have already read more than just the first
-               // few bytes of this file, and more than just a few
-               // bytes of this block, and there's more data for this
-               // file in the next segment/block ... then there's a
-               // good chance we are going to want the next block
-               // soon. Start getting it into the cache now.
+               // few bytes of this file, and we have already
+               // consumed a noticeable portion of this segment, and
+               // there's more data for this file in the next segment
+               // ... then there's a good chance we are going to need
+               // the data for that next segment soon. Start getting
+               // it into the cache now.
                go f.kc.cache().Get(f.kc, f.segNext[0].Locator)
                f.readaheadDone = true
        }
@@ -136,6 +141,7 @@ func (f *file) Read(buf []byte) (int, error) {
        return n, nil
 }
 
+// Seek implements io.Seeker.
 func (f *file) Seek(offset int64, whence int) (int64, error) {
        var want int64
        switch whence {
@@ -158,6 +164,7 @@ func (f *file) Seek(offset int64, whence int) (int64, error) {
        return f.offset, nil
 }
 
+// Len returns the file size in bytes.
 func (f *file) Len() uint64 {
        return uint64(f.size)
 }