[tarantool-patches] Re: [PATCH v3 4/7] iproto: replace obuf by mpstream in execute.c

[Vladimir Davydov]

On Thu, Nov 29, 2018 at 05:04:06PM +0300, Vladislav Shpilevoy wrote:
> On 29/11/2018 13:53, Vladimir Davydov wrote:
> > On Tue, Nov 27, 2018 at 10:25:43PM +0300, imeevma at tarantool.org wrote:
> > > @@ -627,81 +610,53 @@ sql_prepare_and_execute(const struct sql_request *request,
> > >   }
> > >   int
> > > -sql_response_dump(struct sql_response *response, int *keys, struct obuf *out)
> > > +sql_response_dump(struct sql_response *response, int *keys,
> > > +		  struct mpstream *stream)
> > >   {
> > >   	sqlite3 *db = sql_get();
> > >   	struct sqlite3_stmt *stmt = (struct sqlite3_stmt *) response->prep_stmt;
> > > -	struct port_tuple *port_tuple = (struct port_tuple *) &response->port;
> > >   	int rc = 0, column_count = sqlite3_column_count(stmt);
> > >   	if (column_count > 0) {
> > > -		if (sql_get_description(stmt, out, column_count) != 0) {
> > > +		if (sql_get_description(stmt, stream, column_count) != 0) {
> > >   err:
> > >   			rc = -1;
> > >   			goto finish;
> > >   		}
> > >   		*keys = 2;
> > > -		int size = mp_sizeof_uint(IPROTO_DATA) +
> > > -			   mp_sizeof_array(port_tuple->size);
> > > -		char *pos = (char *) obuf_alloc(out, size);
> > > -		if (pos == NULL) {
> > > -			diag_set(OutOfMemory, size, "obuf_alloc", "pos");
> > > -			goto err;
> > > -		}
> > > -		pos = mp_encode_uint(pos, IPROTO_DATA);
> > > -		pos = mp_encode_array(pos, port_tuple->size);
> > > -		/*
> > > -		 * Just like SELECT, SQL uses output format compatible
> > > -		 * with Tarantool 1.6
> > > -		 */
> > > -		if (port_dump_msgpack_16(&response->port, out) < 0) {
> > > +		mpstream_encode_uint(stream, IPROTO_DATA);
> > > +		mpstream_flush(stream);
> > > +		if (port_dump_msgpack(&response->port, stream->ctx) < 0) {
> > 
> > stream->ctx isn't guaranteed to be an obuf
> > 
> > And when you introduce vstream later, you simply move this code to
> > another file. This is confusing. May be we should pass alloc/reserve
> > used in mpstream to port_dump instead of obuf?
> 
> Good idea, though not sure, if it is worth slowing down port_dump_msgpack
> adding a new level of indirection. Since port_dump_msgpack is a hot path
> and is used for box.select.
> 
> Maybe it is better to just rename port_dump_msgpack to port_dump_obuf
> and rename vstream_port_dump to vstream_port_dump_obuf? If we ever will
> dump port to not obuf, then we will just add a new method to port_vtab.
> 
> Also, it would make port_dump_obuf name consistent with port_dump_lua -
> in both cases we not just dump in a specific format, but to a concrete
> destination: obuf and lua stack. Now port_dump_msgpack anyway is restricted
> by obuf destination.

There's port_dump_plain, which dumps port contents in a specific format.
So port_dump_obuf would look ambiguous.

> 
> If you worry about how to call sql_response_dump() to not obuf, then there
> is another option. Anyway rename port_dump_msgpack to port_dump_obuf and
> introduce a new method: port_dump_mpstream. It will take mpstream and use
> its reserve/alloc/error functions. It allows us to do not slow down box.select,
> but use the full power of virtual functions in execute.c, which definitely is
> not hot.

That would interconnect port and mpstream, make them dependent on each
other. I don't think that would be good.

> 
> mpstream implementation of vstream will call port_dump_mpstream, and
> luastream implementation of vstream will call port_dump_lua as it does now.
> box.select and iproto_call will use port_dump_obuf.
> 
> I prefer the second option: introduce port_dump_mpstream. It is ok for you?

I may be wrong, but IMO there isn't much point in optimizing box.select,
because it's very limited in its applicability. People already prefer to
use box.call over box.insert/select/etc over iproto, and with the
appearance of box.execute they are likely to stop using plain box.select
at all.

That said, personally I would try to pass reserve/alloc methods to port,
see how it goes.