From a21232a2558e947e5f9d46de9b9590d8e006050b Mon Sep 17 00:00:00 2001 From: Bin Guo Date: Wed, 3 Jun 2026 10:25:38 +0800 Subject: [PATCH] qobject/json-writer: preallocate output buffer MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit json_writer_new() creates the output GString with g_string_new(NULL), which starts at the GLib default of 64 bytes. Serializing typical QMP responses then requires multiple reallocations as the buffer grows -- for query-qmp-schema the GString is reallocated 12+ times. Preallocate JSON_WRITER_INITIAL_SIZE (4096) bytes. This covers most QMP responses without any reallocation. The JSONWriter is a short-lived object so the preallocation does not accumulate. Reviewed-by: Philippe Mathieu-Daudé Reviewed-by: Markus Armbruster Signed-off-by: Bin Guo Message-ID: <20260603022538.92780-1-guobin@linux.alibaba.com> Signed-off-by: Markus Armbruster --- qobject/json-writer.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/qobject/json-writer.c b/qobject/json-writer.c index aac2c6ab71..c23c81709b 100644 --- a/qobject/json-writer.c +++ b/qobject/json-writer.c @@ -24,13 +24,16 @@ struct JSONWriter { GByteArray *container_is_array; }; +/* Should cover most QMP responses without reallocation */ +#define JSON_WRITER_INITIAL_SIZE 4096 + JSONWriter *json_writer_new(bool pretty) { JSONWriter *writer = g_new(JSONWriter, 1); writer->pretty = pretty; writer->need_comma = false; - writer->contents = g_string_new(NULL); + writer->contents = g_string_sized_new(JSON_WRITER_INITIAL_SIZE); writer->container_is_array = g_byte_array_new(); return writer; }