From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:36248) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Zs7Zl-0005h7-Ix for qemu-devel@nongnu.org; Fri, 30 Oct 2015 07:10:30 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Zs7Zg-0002aX-9u for qemu-devel@nongnu.org; Fri, 30 Oct 2015 07:10:29 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53430) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Zs7Zg-0002aJ-36 for qemu-devel@nongnu.org; Fri, 30 Oct 2015 07:10:24 -0400 From: Gerd Hoffmann Date: Fri, 30 Oct 2015 12:10:14 +0100 Message-Id: <1446203414-4013-20-git-send-email-kraxel@redhat.com> In-Reply-To: <1446203414-4013-1-git-send-email-kraxel@redhat.com> References: <1446203414-4013-1-git-send-email-kraxel@redhat.com> Subject: [Qemu-devel] [PATCH 19/19] buffer: allow a buffer to shrink gracefully List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: pl@kamp.de, Gerd Hoffmann From: Peter Lieven 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 Signed-off-by: Gerd Hoffmann --- include/qemu/buffer.h | 1 + util/buffer.c | 34 ++++++++++++++++++++++++++++------ 2 files changed, 29 insertions(+), 6 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..5461f86 100644 --- a/util/buffer.c +++ b/util/buffer.c @@ -23,6 +23,7 @@ #define BUFFER_MIN_INIT_SIZE 4096 #define BUFFER_MIN_SHRINK_SIZE 65536 +#define BUFFER_AVG_SIZE_SHIFT 7 static size_t buffer_req_size(Buffer *buffer, size_t len) { @@ -37,6 +38,11 @@ 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 +54,30 @@ 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. + 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 << 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 ... */ - if (buffer->offset < (buffer->capacity >> 3) && - buffer->capacity > BUFFER_MIN_SHRINK_SIZE) { - return; + 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 +103,7 @@ uint8_t *buffer_end(Buffer *buffer) void buffer_reset(Buffer *buffer) { buffer->offset = 0; + buffer_shrink(buffer); } void buffer_free(Buffer *buffer) @@ -107,6 +128,7 @@ 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