On Mon, Aug 12, 2013 at 03:07:47PM -0700, Mengmeng Chen wrote:
> Hi,
>
> I'm reading the source code of MonetDB and I have a question about
> multi-threaded BAT binding and would appreciate it if someone could
> help to answer.
>
> Here is my question:
>
> When a sql_bind() MAL instruction is called, I see we have a callstack
> something like:
>
> mvc_bind_wrap -> mvc_bind-> mvc_bind_column -> find_sql_column ->
> _cs_find_name->_list_find_name
>
> In _list_find_name, we have something like:
>
>     if (l && !l->ht && list_length(l) > HASH_MIN_SIZE && l->sa) {
>         l->ht = hash_new(l->sa, list_length(l), (fkeyvalue)&base_key);
>
>         for (n = l->h; n; n = n->next ) {
>             sql_base *b = n->data;
>             int key = base_key(b);
>
>             hash_add(l->ht, key, b);
>         }
>     }
>     if (l && l->ht) {
>         int key = hash_key(name);
>         sql_hash_e *he = l->ht->buckets[key&(l->ht->size-1)];
>
>         for (; he; he = he->chain) {
>             sql_base *b = he->value;
>
>             if (b->name && strcmp(b->name, name) == 0)
>                 return b;
>         }
>         return NULL;
>     }
>
>     if (l)
>         for (n = l->h; n; n = n->next) {
>             sql_base *b = n->data;
>
>             /* check if names match */
>             if (name[0] == b->name[0] && strcmp(name, b->name) == 0) {
>                 return b;
>             }
>         }
>
> Looks like the function is trying to create a hash table for the list
> if called the first time. However, there is no protection against data
> contention. Now suppose the mitosis optimizer is turned on and we could
> have something like:
>
>  X_157:bat[:oid,:oid]  := sql.tid(X_4,"sys","lineitem",0,8);
>  X_159:bat[:oid,:oid]  := sql.tid(X_4,"sys","lineitem",1,8);
>  X_161:bat[:oid,:oid]  := sql.tid(X_4,"sys","lineitem",2,8);
>  X_163:bat[:oid,:oid]  := sql.tid(X_4,"sys","lineitem",3,8);
>  X_165:bat[:oid,:oid]  := sql.tid(X_4,"sys","lineitem",4,8);
>  X_167:bat[:oid,:oid]  := sql.tid(X_4,"sys","lineitem",5,8);
>  X_169:bat[:oid,:oid]  := sql.tid(X_4,"sys","lineitem",6,8);
>  X_171:bat[:oid,:oid]  := sql.tid(X_4,"sys","lineitem",7,8);
>
> If my understanding is correct, since these MAL instructions has no
> dependency in each other, they could potentially be executed in
> different threads simultaneously. How would MonetDB prevent data
> contention in _list_find_name()?  Particularly the case that if thread
> A has created a hash table and has not yet inserted the elements while
> thread B tries to search through the list and thinks the hash table is
> ready and starts to use it. It will fail to return the correct column
> since the hash table is empty, right?
>

If the hash is created during queriing, your conclusion is right. But
that isn't the case, the hashtable gets created during table create,
table loading (server startup) and plan creation. Al these aren't
concurrent.

Niels
>
>
> Thanks,
> Mengmeng

> _______________________________________________
> users-list mailing list
> users-list@monetdb.org
> http://mail.monetdb.org/mailman/listinfo/users-list


--
Niels Nes, Centrum Wiskunde & Informatica (CWI)
Science Park 123, 1098 XG Amsterdam, The Netherlands
room L3.14,  phone ++31 20 592-4098     sip:4098@sip.cwi.nl
url: http://www.cwi.nl/~niels   e-mail: Niels.Nes@cwi.nl

_______________________________________________
users-list mailing list
users-list@monetdb.org
http://mail.monetdb.org/mailman/listinfo/users-list