Re: [Tarantool-patches] [PATCH 03/16] tx: introduce dirty tuples

[Nikita Pettik <korablev@tarantool.org>]

On 12 Jul 19:15, Vladislav Shpilevoy wrote:
> I still have the same question as in the previous review:
> 
> 	How are the dirty tuples going to be used? Why don't we have such a flag
> 	for vinyl tuples?
>

Alexander a bit explained me how new engine is supposed to work.
I believe he is going to publish sort of docs containing details soon.
As I understand, one of the main goals of new engine is to avoid
performance degradation in case tuple is not modified during transaction.
'dirty' tuple is a tuple which features 'story': we have to seek through
changes to find the proper one (depending on existing read views etc).
'clean' tuple is a casual tuple: if we read such tuple we return immediately.
 
> > diff --git a/src/box/tuple.h b/src/box/tuple.h
> > index 9a88772..4752323 100644
> > --- a/src/box/tuple.h
> > +++ b/src/box/tuple.h
> > @@ -319,7 +319,13 @@ struct PACKED tuple
> >  	/**
> >  	 * Offset to the MessagePack from the begin of the tuple.
> >  	 */
> > -	uint16_t data_offset;
> > +	uint16_t data_offset : 15;
> > +	/**
> > +	 * The tuple (if it's found in index for example) could be invisible
> > +	 * for current transactions. The flag means that the tuple must
> > +	 * be clarified by transaction engine.
> 
> What is current transaction? Assume, we can yield now. And there are
> some transactions. A new transaction appears and adds a tuple to an index.
> Will be it be visible for newer transactions? Since they are not 'current'
> in terms of when the tuple was added. Or will it be visible to this
> transaction only?
> 
> > +	 */
> > +	bool is_dirty : 1;
> >  	/**
> >  	 * Engine specific fields and offsets array concatenated
> >  	 * with MessagePack fields array.
> > @@ -1081,8 +1087,10 @@ tuple_unref(struct tuple *tuple)
> >  	assert(tuple->refs - 1 >= 0);
> >  	if (unlikely(tuple->is_bigref))
> >  		tuple_unref_slow(tuple);
> > -	else if (--tuple->refs == 0)
> > +	else if (--tuple->refs == 0) {
> > +		assert(!tuple->is_dirty);
> 
> Why? Is something supposed to clear this flag after it is set?
> 
> >  		tuple_delete(tuple);
> > +	}
> >  }