Bin Guo <[email protected]> writes: > 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.
That's an extreme case. Most responses are *much* smaller. Still, starting with a larger buffer makes sense. > 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. I doubt "one page" matters. How many QMP commands get executed in practice? A couple of hundred during startup, then tens per second? Probably less than that. > The JSONWriter > is a short-lived object so the preallocation does not accumulate. > > Signed-off-by: Bin Guo <[email protected]> > --- > 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) */ Covering most responses matters, one page does not. Suggest /* 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; > } Consider tweaking the commit message and the comment to address my remarks. Reviewed-by: Markus Armbruster <[email protected]>
