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

阿四 ashi08104 at gmail.com
Thu Apr 5 09:44:40 CDT 2012


2012/4/4 Gedare Bloom <gedare at rtems.org>

> On Wed, Apr 4, 2012 at 10:42 AM, 阿四 <ashi08104 at gmail.com> wrote:
> > 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?
> I don't think you can make such an assumption since unlimited objects
> are not statically determined. As you point out the space wasted would
> be quite large.
>
I see.


> >
> > 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?
> You mean to use a hash-array with an rbtree to handle collisions?

Yeah, I mean it.

> It
> could work, and be faster on average than just using an rbtree without
> losing any worst-case time. The space overhead might be quite a bit
> more to have multiple rbtree control headers and also the extra
> overhead of rbtree nodes. I think that any rbtree- or chain-based
> approach shifts the problem of determining the number of Keys to be
> the problem of determining the number of nodes, since we cannot assume
> that the Key data has a node embedded inside of it.
>
> Gedare, I didn't quite understand this. Does rbtree approach has the
problem of determining the number of nodes? I think when the key_create()
function called, only a empty rbtree create. And when key_setspecific()
function called, a node is added to rbtree. Or you mean create a 'proper'
big rbtree when key_create(), which frontloads the insert node cost to
key_create() as you mention above? I understand when using the hash
approach, determing the number of slots(I'm confused with the Keys you use
here, I guess Keys here is the slots in hash table) of hash table is the
problem of determining the number of nodes.

I don't see why we don't just put a pointer to the Key in the tcb.
>
Does it mean delete the POSIX Key data structure, and let the Thread
Specific Data be managed in TCB?

>
> -Gedare
>
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