Re: [tarantool-patches] Re: [PATCH v4 2/5] iproto: replace obuf by mpstream in execute.c

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Mon, Dec 03, 2018 at 11:48:26PM +0300, Vladislav Shpilevoy wrote:
> 
> 
> On 03/12/2018 18:21, Vladimir Davydov wrote:
> > On Sun, Dec 02, 2018 at 02:03:21PM +0300, imeevma@tarantool.org wrote:
> > > This patch is the most dubious patch due to the implicit use of
> > > mpstream as a stream for obuf. Discussion and patch below.
> > > 
> > > It is worth noting that in this version of the patch nothing
> > > changes. At this point there is no approved solution for this
> > > patch.
> > > 
> > > 
> > > On 11/30/18 1:55 PM, Vladimir Davydov wrote:
> > > > On Fri, Nov 30, 2018 at 01:45:48PM +0300, Vladislav Shpilevoy wrote:
> > > > > 
> > > > > 
> > > > > On 30/11/2018 13:19, Vladimir Davydov wrote:
> > > > > > 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@tarantool.org wrote:
> > > > > > > > > @@ -625,81 +608,53 @@ 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.
> > > > > > 
> > > > > 
> > > > > I do not see a reason to slow down box.select if we can don't do it.
> > > > > Yeas, people use IPROTO_CALL, but in stored functions they use box
> > > > > functions including select.
> > > > 
> > > > box.select called from Lua code doesn't use port_dump_msgpack.
> > > > 
> > > > > 
> > > > > Ok, instead of port_dump_mpstream we can rename port_dump_msgpack to
> > > > > port_dump_obuf and add port_dump_msgpack which does not depend on
> > > > > mpstream and takes alloc/reserve/ctx directly.
> > > > 
> > > > Better call the optimized version (the one without callbacks)
> > > > port_dump_msgpack_obuf to avoid confusion IMO.
> > > > 
> > > > Anyway, I'd try to run cbench to see if it really perfomrs better
> > > > than the one using callbacks.
> > > 
> > > @@ -625,81 +608,53 @@ 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) {
> > 
> > Still, I'm quite convinced that we need to pass alloc/reserve methods
> > along with ctx to port_dump_msgpack(), because implicitly assumping that
> > mpstream->ctx is, in fact, an obuf looks very fragile. However, Vlad is
> > right that it may indeed affect performance in a negative way. So let's
> > perhaps do the following:
> > 
> >   1. Run cbench to see how badly indirect obuf_alloc/reserve slows
> >      things down.
> > 
> >   2. Consider the possibility of using templates or macro definitions
> >      instead of function pointers.
> > 
> > What do you think?
> > 
> 
> Good plan except one thing in its second point: port still must feature
> double-virtualized method taking alloc/reserve to be "dumpable" via
> mpstream. Yes, we can leave obuf method, even add region dump method in
> future, but for mpstream it requires virtual alloc/reserve anyway
> (until mpstream is templated). My point is in saving every single
> percent of perf for calls and selects. For SQL alloc/reserve is enough.

Anyway, it'd be nice to see how much it's going to save, exactly.

> 
> What about bench - yes, maybe it is worth benching double-virtualized
> port vs specialized. It should test calls and selects. But one problem -
> as I know, cbench does not use iproto but port_dump_msgpack is reachable
> from iproto only.

Let's try nosqlbench then.