qobject/json-writer: preallocate output buffer
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>
Bin Guo committed
Jun 3, 2026 at 10:25 UTC
a21232a2558e947e5f9d46de9b9590d8e006050b
1 file changed
+4
-1
qobject/json-writer.c
+4
-1
@@ -24,13 +24,16 @@ struct JSONWriter {
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GByteArray *container_is_array;
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};
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+/* Should cover most QMP responses without reallocation */
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+#define JSON_WRITER_INITIAL_SIZE 4096
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+
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JSONWriter *json_writer_new(bool pretty)
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{
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JSONWriter *writer = g_new(JSONWriter, 1);
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writer->pretty = pretty;
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writer->need_comma = false;
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- writer->contents = g_string_new(NULL);
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+ writer->contents = g_string_sized_new(JSON_WRITER_INITIAL_SIZE);
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writer->container_is_array = g_byte_array_new();
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return writer;
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}