On Feb 27, 6:48 pm, Mikael Johansson <[email protected]> wrote:
> Hi,
>
> The 2.2.x branch had the same performance improvement but it wasn't
> released until a few minutes ago :) Try the 2.2.5 release, should give
> you better performance when using the consistent hashing strategy.
The performance improvement people are talking about is:
http://cvs.php.net/viewvc.cgi/pecl/memcache/memcache_consistent_hash.c?r1=1.6&r2=1.7
it might bring you some performance improvement in the algorithm
selecting a server for a particular key. Nevertheless, the
implementation is slow and clumsy: it creates <weight> * 160 hashes
per server and then takes 1024 of them. One of these 1024 is chosen
upon a simple division <your hash> % 1024, which points back to a
server from your pool.
In the case of the standard algorithm, you create an array in which
every server contributes <weight> elements. You decide upon a server
again by a simple division <your key> % <array size>.
As you can see, the consistent algorithm has a great performance
impact due to:
- calculating <weight> * 160 hashes for each server
- choosing 1024 out of all hashes
another negative impact, that you didn't saw is when you have a small
number of servers (3 - 4) and small weights (say 1), then the load on
the servers is unequally distributed, due to the fact that you have to
fill these 1024 places with say 640 elements. As the filling algorithm
works, some elements will take several places in the 1024 array,
therefore upon server selection via <key> % 1024, some servers will
have higher probability to be selected.
Having said this, if you need performance improvements:
- try the patch mentioned above
- try some other php client
- look how others do server selection and implement it in some
present php client
- design your own server selection algorithm and implement it
- stick with the standard straight forward algorithm
* The constants mentioned above are to be found in
http://cvs.php.net/viewvc.cgi/pecl/memcache/php_memcache.h?revision=1.39&view=markup
namely:
90 #define MMC_CONSISTENT_POINTS 160 /* points per
server */
91 #define MMC_CONSISTENT_BUCKETS 1024 /* number of
precomputed
buckets, should be power of 2 */
