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.  4096 is one page on
most systems, which is efficient for the allocator.  The JSONWriter
is a short-lived object so the preallocation does not accumulate.

Signed-off-by: Bin Guo <guobin@linux.alibaba.com>
---
 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..fb3f3f3e3c 100644
--- a/qobject/json-writer.c
+++ b/qobject/json-writer.c
@@ -24,13 +24,16 @@ struct JSONWriter {
     GByteArray *container_is_array;
 };
 
+/* Covers most QMP responses without reallocation (one page) */
+#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)

On 29/5/26 04:34, Bin Guo wrote:
> 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.  4096 is one page on
> most systems, which is efficient for the allocator.

Why not use qemu_real_host_page_size()?

Anyhow,
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

>  The JSONWriter
> is a short-lived object so the preallocation does not accumulate.
> 
> Signed-off-by: Bin Guo <guobin@linux.alibaba.com>
> ---
>   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..fb3f3f3e3c 100644
> --- a/qobject/json-writer.c
> +++ b/qobject/json-writer.c
> @@ -24,13 +24,16 @@ struct JSONWriter {
>       GByteArray *container_is_array;
>   };
>   
> +/* Covers most QMP responses without reallocation (one page) */
> +#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;
>   }