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qobject/json-writer: preallocate output buffer

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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>
Message-ID: <20260603022538.92780-1-guobin@linux.alibaba.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
This commit is contained in:
Bin Guo
2026-06-03 10:25:38 +08:00
committed by Markus Armbruster
parent a59157f98f
commit a21232a255
1 changed files with 4 additions and 1 deletions
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5
qobject/json-writer.c
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@@ -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;
}