On Mon, Aug 18, 2014 at 7:01 PM, Greg Freemyer <greg.freemyer@xxxxxxxxx> wrote: > On Mon, Aug 18, 2014 at 4:20 PM, Nick Krause <xerofoify@xxxxxxxxx> wrote: >> What are the advantages of the hashed linked list version over the >> standard one and does it >> increase the memory usage and overhead of the linked list more if I >> use a hashed version? > > Seriously? Do you know what a hash is? > > A hash is a well-defined many to one algorithm. > > If I have a universe of a million items that hash down to 100 unique > hashes, then I can group those million items by hash and have 100 > groups of roughly 10,000 items each. > > The better the hashing algorithm versus my original universe of 1 > million items, the more even the distribution. > > Now that I have 100 segregated groups I can build an array of 100 > linked lists all maintained separately. > > Thus: > > hash_index = my_hash(item) > > add_item(linked_list[hash_item], item) is how I add my item to the > hashed linked list. > > is_in_list(linked_list[hash_item], item) is how I check to see if my > item is already in the list. > > So in my example I have to have 100 linked lists, but each list is on > average 100x smaller than a simple linked list would be. > > Is adding an item to the hashed linked list faster? > > Absolutely not, I have to hash the item first then do a normal linked > list insertion. That will always be slower. > > Is finding the item faster? > > That is the whole point of the exercise. The theory is you ONLY use a > hashed linked list if the overhead of hashing the item is less than > the amount of time saved by traversing shorter lists when you search. > > It is the job of the programmer to make the determination if a hashed > list is a better choice or not on a case by case basis. It depends on > the length of the list without breaking it into pieces and how well > the hash algorithm can do at generating roughly similar segregated > groups. > > For the size question, write yourself a userspace app and test it. > Obviously that is more work than asking here, but it is ASSUMED you > are doing research on your OWN before you post questions here. > > fyi: this question has little to do with the linux kernel. It is part > of what people mean when they say you need to go learn c before you > start on the kernel. Using linked lists and hashed linked lists is > stuff you can fully explore in userspace. > > Greg No I known what the advantages are for user space was wondering if there were any issues that differ in kernel space. Nick _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
