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[Qemu-devel] [PATCH 1/2] io/buffer: allow a buffer to shrink gracefully
From: |
Peter Lieven |
Subject: |
[Qemu-devel] [PATCH 1/2] io/buffer: allow a buffer to shrink gracefully |
Date: |
Tue, 3 Nov 2015 10:01:15 +0100 |
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>
---
include/io/buffer.h | 1 +
io/buffer.c | 42 +++++++++++++++++++++++++++++++++---------
2 files changed, 34 insertions(+), 9 deletions(-)
diff --git a/include/io/buffer.h b/include/io/buffer.h
index f6668cb..f63869e 100644
--- a/include/io/buffer.h
+++ b/include/io/buffer.h
@@ -37,6 +37,7 @@ struct QIOBuffer {
char *name;
size_t capacity;
size_t offset;
+ uint64_t avg_size;
uint8_t *buffer;
};
diff --git a/io/buffer.c b/io/buffer.c
index 0fd3cea..d2a6043 100644
--- a/io/buffer.c
+++ b/io/buffer.c
@@ -23,6 +23,10 @@
#define QIO_BUFFER_MIN_INIT_SIZE 4096
#define QIO_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 QIO_BUFFER_AVG_SIZE_SHIFT 7
static size_t buf_req_size(QIOBuffer *buffer, size_t len)
{
@@ -37,6 +41,11 @@ static void buf_adj_size(QIOBuffer *buffer, size_t len)
buffer->buffer = g_realloc(buffer->buffer, buffer->capacity);
trace_qio_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 << QIO_BUFFER_AVG_SIZE_SHIFT);
}
void qio_buffer_init(QIOBuffer *buffer, const char *name, ...)
@@ -48,21 +57,34 @@ void qio_buffer_init(QIOBuffer *buffer, const char *name,
...)
va_end(ap);
}
-void qio_buffer_shrink(QIOBuffer *buffer)
+static uint64_t get_buf_avg_size(QIOBuffer *buffer)
{
- /*
- * Only shrink in case the used size is *much* smaller than the
- * capacity, to avoid bumping up & down the buffers all the time.
+ return buffer->avg_size >> QIO_BUFFER_AVG_SIZE_SHIFT;
+}
+
+void qio_buffer_shrink(QIOBuffer *buffer)
+ {
+ 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 ^ QIO_BUFFER_AVG_SIZE_SHIFT. */
+ buffer->avg_size *= (1 << QIO_BUFFER_AVG_SIZE_SHIFT) - 1;
+ buffer->avg_size >>= QIO_BUFFER_AVG_SIZE_SHIFT;
+ buffer->avg_size += buf_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 ...
*/
- if (buffer->offset < (buffer->capacity >> 3) &&
- buffer->capacity > QIO_BUFFER_MIN_SHRINK_SIZE) {
- return;
+ new = buf_req_size(buffer, get_buf_avg_size(buffer));
+ if (new < buffer->capacity >> 3 &&
+ new >= QIO_BUFFER_MIN_SHRINK_SIZE) {
+ buf_adj_size(buffer, get_buf_avg_size(buffer));
}
-
- buf_adj_size(buffer, 0);
}
+
void qio_buffer_reserve(QIOBuffer *buffer, size_t len)
{
if ((buffer->capacity - buffer->offset) < len) {
@@ -83,6 +105,7 @@ uint8_t *qio_buffer_end(QIOBuffer *buffer)
void qio_buffer_reset(QIOBuffer *buffer)
{
buffer->offset = 0;
+ qio_buffer_shrink(buffer);
}
void qio_buffer_free(QIOBuffer *buffer)
@@ -107,6 +130,7 @@ void qio_buffer_advance(QIOBuffer *buffer, size_t len)
memmove(buffer->buffer, buffer->buffer + len,
(buffer->offset - len));
buffer->offset -= len;
+ qio_buffer_shrink(buffer);
}
void qio_buffer_move_empty(QIOBuffer *to, QIOBuffer *from)
--
1.9.1