buffer: allow a buffer to shrink gracefully

the idea behind this patch is to allow the buffer to shrink, but
make this a seldom operation. The buffers average size is measured
exponentionally smoothed with am alpha of 1/128.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1446203414-4013-20-git-send-email-kraxel@redhat.com

include/qemu/buffer.h

@@ -37,6 +37,7 @@ struct Buffer {
     char *name;
     size_t capacity;
     size_t offset;
+    uint64_t avg_size;
     uint8_t *buffer;
 };
 

util/buffer.c

@@ -24,6 +24,11 @@
 #define BUFFER_MIN_INIT_SIZE     4096
 #define BUFFER_MIN_SHRINK_SIZE  65536
 
+/* define the factor alpha for the expentional smoothing
+ * that is used in the average size calculation. a shift
+ * of 7 results in an alpha of 1/2^7. */
+#define BUFFER_AVG_SIZE_SHIFT       7
+
 static size_t buffer_req_size(Buffer *buffer, size_t len)
 {
     return MAX(BUFFER_MIN_INIT_SIZE,
@@ -37,6 +42,11 @@ static void buffer_adj_size(Buffer *buffer, size_t len)
     buffer->buffer = g_realloc(buffer->buffer, buffer->capacity);
     trace_buffer_resize(buffer->name ?: "unnamed",
                         old, buffer->capacity);
+
+    /* make it even harder for the buffer to shrink, reset average size
+     * to currenty capacity if it is larger than the average. */
+    buffer->avg_size = MAX(buffer->avg_size,
+                           buffer->capacity << BUFFER_AVG_SIZE_SHIFT);
 }
 
 void buffer_init(Buffer *buffer, const char *name, ...)
@@ -48,16 +58,29 @@ void buffer_init(Buffer *buffer, const char *name, ...)
     va_end(ap);
 }
 
+static uint64_t buffer_get_avg_size(Buffer *buffer)
+{
+    return buffer->avg_size >> BUFFER_AVG_SIZE_SHIFT;
+}
+
 void buffer_shrink(Buffer *buffer)
 {
-    /*
-     * Only shrink in case the used size is *much* smaller than the
-     * capacity, to avoid bumping up & down the buffers all the time.
-     * realloc() isn't exactly cheap ...
-     */
-    if (buffer->offset < (buffer->capacity >> 3) &&
-        buffer->capacity > BUFFER_MIN_SHRINK_SIZE) {
-        return;
+    size_t new;
+
+    /* Calculate the average size of the buffer as
+     * avg_size = avg_size * ( 1 - a ) + required_size * a
+     * where a is 1 / 2 ^ BUFFER_AVG_SIZE_SHIFT. */
+    buffer->avg_size *= (1 << BUFFER_AVG_SIZE_SHIFT) - 1;
+    buffer->avg_size >>= BUFFER_AVG_SIZE_SHIFT;
+    buffer->avg_size += buffer_req_size(buffer, 0);
+
+    /* And then only shrink if the average size of the buffer is much
+     * too big, to avoid bumping up & down the buffers all the time.
+     * realloc() isn't exactly cheap ...  */
+    new = buffer_req_size(buffer, buffer_get_avg_size(buffer));
+    if (new < buffer->capacity >> 3 &&
+        new >= BUFFER_MIN_SHRINK_SIZE) {
+        buffer_adj_size(buffer, buffer_get_avg_size(buffer));
     }
 
     buffer_adj_size(buffer, 0);
@@ -83,6 +106,7 @@ uint8_t *buffer_end(Buffer *buffer)
 void buffer_reset(Buffer *buffer)
 {
     buffer->offset = 0;
+    buffer_shrink(buffer);
 }
 
 void buffer_free(Buffer *buffer)
@@ -107,6 +131,7 @@ void buffer_advance(Buffer *buffer, size_t len)
     memmove(buffer->buffer, buffer->buffer + len,
             (buffer->offset - len));
     buffer->offset -= len;
+    buffer_shrink(buffer);
 }
 
 void buffer_move_empty(Buffer *to, Buffer *from)