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author | Peter Lieven <pl@kamp.de> | 2015-10-30 12:10:14 +0100 |
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committer | Gerd Hoffmann <kraxel@redhat.com> | 2015-11-05 09:09:58 +0100 |
commit | f14c3d85b003d8614144ae67a26157667c1e1245 (patch) | |
tree | 0ffdab65f95a165d8a9000351ac0e22364cf39e8 /util/buffer.c | |
parent | 4ec5ba151ff3f2ac8dc44dabd058eca5846654a6 (diff) | |
download | qemu-f14c3d85b003d8614144ae67a26157667c1e1245.zip qemu-f14c3d85b003d8614144ae67a26157667c1e1245.tar.gz qemu-f14c3d85b003d8614144ae67a26157667c1e1245.tar.bz2 |
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
Diffstat (limited to 'util/buffer.c')
-rw-r--r-- | util/buffer.c | 41 |
1 files changed, 33 insertions, 8 deletions
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) |