caching: Optimize memory allocations. (#3405)

This change brings in changes at multiple places

 - Reuse buffers at almost all locations ranging
   from rpc, fs, xl, checksum etc.
 - Change caching behavior to disable itself
   under low memory conditions i.e < 8GB of RAM.
 - Only objects cached are of size 1/10th the size
   of the cache for example if 4GB is the cache size
   the maximum object size which will be cached
   is going to be 400MB. This change is an
   optimization to cache more objects rather
   than few larger objects.
 - If object cache is enabled default GC
   percent has been reduced to 20% in lieu
   with newly found behavior of GC. If the cache
   utilization reaches 75% of the maximum value
   GC percent is reduced to 10% to make GC
   more aggressive.
 - Do not use *bytes.Buffer* due to its growth
   requirements. For every allocation *bytes.Buffer*
   allocates an additional buffer for its internal
   purposes. This is undesirable for us, so
   implemented a new cappedWriter which is capped to a
   desired size, beyond this all writes rejected.

Possible fix for #3403.

cmd/storage-rpc-server.go

@@ -17,7 +17,6 @@
 package cmd
 
 import (
-	"bytes"
 	"io"
 	"net/rpc"
 	"path"
@@ -156,19 +155,12 @@ func (s *storageServer) ReadAllHandler(args *ReadFileArgs, reply *[]byte) error
 
 // ReadFileHandler - read file handler is rpc wrapper to read file.
 func (s *storageServer) ReadFileHandler(args *ReadFileArgs, reply *[]byte) (err error) {
-	defer func() {
-		if r := recover(); r != nil {
-			// Recover any panic and return ErrCacheFull.
-			err = bytes.ErrTooLarge
-		}
-	}() // Do not crash the server.
 	if !isRPCTokenValid(args.Token) {
 		return errInvalidToken
 	}
-	// Allocate the requested buffer from the client.
-	*reply = make([]byte, args.Size)
+
 	var n int64
-	n, err = s.storage.ReadFile(args.Vol, args.Path, args.Offset, *reply)
+	n, err = s.storage.ReadFile(args.Vol, args.Path, args.Offset, args.Buffer)
 	// Sending an error over the rpc layer, would cause unmarshalling to fail. In situations
 	// when we have short read i.e `io.ErrUnexpectedEOF` treat it as good condition and copy
 	// the buffer properly.
@@ -176,7 +168,7 @@ func (s *storageServer) ReadFileHandler(args *ReadFileArgs, reply *[]byte) (err
 		// Reset to nil as good condition.
 		err = nil
 	}
-	*reply = (*reply)[0:n]
+	*reply = args.Buffer[0:n]
 	return err
 }