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

[Vladislav Shpilevoy]

On 03/12/2018 18:21, Vladimir Davydov wrote:
> On Sun, Dec 02, 2018 at 02:03:21PM +0300, imeevma at 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 at 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.

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.