[GSoC]use hash/map in POSIX key and Classic notepad

阿四 ashi08104 at gmail.com
Wed Apr 4 14:42:54 UTC 2012

Sorry for reply late! I spent some time to take a close look at hash and
rbtree. Thanks for your idea!

On 2/04/12 4:21 AM, Joel Sherrill wrote:
>>  From phone. I don't know how using rbtree would impact memory allocation
>> in the sense of when allocations occur but since if the tree stays
>> balanced, that may be a viable implementation.
>> Or a hash... I know the problem and don't claim to know the perfect
>> solution. :)
> But if you use thread ids as keys, you have known properties on the values
>> being hashed or indexed.
Yes, I realise use pthread_key_t as the values being hashed is wrong.
Thread ids should be appropriate.

> Maybe this is something the project could focus on. Looking at both and
> seeing how they stack up against each other.

Using the rbtree you don't need a hash since you can use thread ids
> directly (I think?); using a hash you still need some structure to map
> into for insert/search of key data, such as a tree or an array of
> linked lists. The time complexity of the different map algorithms is
> what matters most here. The usual hash+array solution is theoretically
> quite bad in the worst case; leveraging properties about the threads
> themselves might make it better. One problem that I see with using an
> rbtree (or array+chain) is that the Key.Values will need to embed an
> rbtree_node (or chain_node) along with the user data (void*).
> A question about the time complexity of map algorithms in hash table: can
we make any assumption about the number of Threads or Tasks that RTEMS can
create when POSIX threads are configured as "unlimited"? Say the number
can't be larger than some value. Then a hash talbe with enough slot can be
created, and the runtime of map algorithm here may be not very important.
However, if the hash table is too big, the problem of memory waste comes
again. But I wonder compared to the exatra reserved memory in current
implement of POSIX key, is memory waste of big hash table a big probem?

And another idea of hash + map, if the map part is a sub rbtree, then the
size of hash table can be not too big, the search operation could benifit
from the rbtree's O(lg(n)) speed, could it be a candidate of the solution?
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