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[Qemu-devel] [PULL 19/20] buffer: allow a buffer to shrink gracefully
From: |
Gerd Hoffmann |
Subject: |
[Qemu-devel] [PULL 19/20] buffer: allow a buffer to shrink gracefully |
Date: |
Mon, 16 Nov 2015 18:25:34 +0100 |
From: Peter Lieven <address@hidden>
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 <address@hidden>
Signed-off-by: Gerd Hoffmann <address@hidden>
Reviewed-by: Daniel P. Berrange <address@hidden>
Message-id: address@hidden
---
include/qemu/buffer.h | 1 +
util/buffer.c | 41 +++++++++++++++++++++++++++++++++--------
2 files changed, 34 insertions(+), 8 deletions(-)
diff --git a/include/qemu/buffer.h b/include/qemu/buffer.h
index 0a69b3a..dead9b7 100644
--- a/include/qemu/buffer.h
+++ b/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;
};
diff --git a/util/buffer.c b/util/buffer.c
index fe5a44e..8b27c08 100644
--- a/util/buffer.c
+++ b/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)
--
1.8.3.1
- [Qemu-devel] [PULL 13/20] vnc: use vnc_{width, height} in vnc_set_area_dirty, (continued)
- [Qemu-devel] [PULL 13/20] vnc: use vnc_{width, height} in vnc_set_area_dirty, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 01/20] buffer: make the Buffer capacity increase in powers of two, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 09/20] vnc-jobs: move buffer reset, use new buffer move, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 04/20] buffer: add buffer_move, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 10/20] vnc: zap dead code, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 06/20] buffer: add tracing, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 08/20] vnc: kill jobs queue buffer, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 02/20] buffer: add buffer_init, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 18/20] buffer: factor out buffer_adj_size, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 16/20] vnc: recycle empty vs->output buffer, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 19/20] buffer: allow a buffer to shrink gracefully,
Gerd Hoffmann <=
- [Qemu-devel] [PULL 11/20] vnc: add vnc_width+vnc_height helpers, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 03/20] buffer: add buffer_move_empty, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 14/20] vnc: only alloc server surface with clients connected, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 05/20] buffer: add buffer_shrink, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 20/20] vnc: fix mismerge, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 07/20] vnc: attach names to buffers, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 17/20] buffer: factor out buffer_req_size, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 12/20] vnc: factor out vnc_update_server_surface, Gerd Hoffmann, 2015/11/16
- [Qemu-devel] [PULL 15/20] vnc: fix local state init, Gerd Hoffmann, 2015/11/16
- Re: [Qemu-devel] [PULL for-2.5 00/20] vnc: buffer code improvements, bugfixes., Peter Maydell, 2015/11/17