See:
          Description

The "PG2K" Java/servlet "dynamic" implementation of the DHD Multimedia Gallery.

The DHD Multimedia Gallery exists as a "pro bono" source of multimedia for people who cannot produce their own or who cannot use a commercial provider. The Gallery started (as the DHD Photo Gallery) when digital cameras and scanners were relatively expensive, and so was a rare source of free/cheap digital Web graphics.

The Gallery now has increasingly varied and comprehensive company in the genuinely-cheap/free multimedia area, which is very good news from everyone from schoolkids to Web masters!

PG2K replaces (and in some cases steals the code from) an earlier incarnation (well two: one based on shell scripts and ImageMagick, and a later variant that replaced most of the scripts with Java and wrote the entire Gallery out as a series of static HTML files (and pre-generated thumbnails, etc) so that it could be served by an ordinary Apache Web server with no CGI). The earlier "static" mechanism was very efficient and safe to serve but was starting to take more than 24 hours to rebuild, even incrementally, and had limited flexibility. It was also repeatedly overrun by greedy spiders/robots opening tens of concurrent connections each and bringing down the whole system and my expensive Internet link.

The servlet version of the Gallery has allowed the distribution to live replicas geographically distributed (in the US, UK, AU and IN as at November 2009), running on fairly cheap Linux and Solaris boxes from circa 1000MIPS/512MB capacity up to multi-CPU/multi-GB/multi-GHz boxes.

PG2K uses some live geo-sensitive load-balancing, has some i18n support, and a few interesting projects cooking (see the to-do list.)

With JDK 1.5+, heavy use has been made of the the new JSR-166 concurrency utilities to improve throughput, especially RentrantLock and tryLock() and the reader/writer lock in the ExhibitDataSimpleCache, and the Semaphore in ExhibitDataHTTPTunnelSource. Also, use has been made of the improved semantics of volatile, and of the AtomicXXX types, to convert some code to being lockless. An advantage of lockless synchronisation is that it may be more efficient, but in any case it is deadlock-free!

Much effort as at the start of 2006 had been put into improving concurrency to take advantage of the many cores available in the Gallery's new T1000/Niagara/CoolThreads server, and to reduce page latency for users.

At the turn of 2008 much effort has been put into making this Gallery run well in power- (and thus CPU- and IO-) constrained circumstances, so that the master server can run on a ~20W laptop partially powered by renewable energy, and Gallery slaves can be set to reduce power consumption at times of local grid peak demand.

From mid-to-end 2009 further effort was put into reducing memory footprint to allow the possibility of running on a ~5W 'embedded' ARMv5 Linux 1.2GHz/512MB "SheevaPlug" system at the low-end, while still making good use of available resources on the larger hosts. A great deal of attention has had to be paid to trimming data structure footprint, releasing memory when unnecessary (and in emergency), efficiency and lazy evaluation, and in general adapting memory and CPU use dynamically to fit available resources. The net result is that the SheevaPlug master can run in under 4W from off-grid solar power while also supporting other services such as email, public NTP, and other Web sites.