[PATCH v2] qobject/json-writer: preallocate output buffer

[Bin Guo <guobin@linux.alibaba.com>]

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 <philmd@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Bin Guo <guobin@linux.alibaba.com>
---
v1->v2: drop "one page" argument from commit message and comment
  (per Markus Armbruster's review)

 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;
 }
-- 
2.50.1 (Apple Git-155)