Implementing memcached in OpenGuides

I’ve just implemented a fair amount of memcached support into CGI::Wiki, the Wiki software that powers OpenGuides. Now, any time a node is fetched, it will come from memcached if possible, or be loaded and stored into memcached, and when nodes are edited, the cached version is deleted.

This is in addition to adding caching to the main map pages, which were previously taking up to 20 seconds to load, and are now in the ~1 sec/range. (There is much more slowness in the browser side of this page, due to the javascript proccessing time required to load hundreds of location markers).

Additionally, the Open Guide to Boston is now running under mod_perl after my Message to the OG-Dev list with a patch that fixed the major stumbling block. This has increased speed somewhere between 5 and 10 fold over using CGI.

About 15 minutes ago, I restarted memcached to clear out old stats. I then visited the main index pages for both the Milton Keynes and Boston guides, to prefill the caches. There were 3535 items which were loaded (GET\_MISSes). In that time, there have been 4 new objects added — and 946 GET\_HITs. With the stability of memcached, I expect that this cache will be able to run for a long time with no maintenance, and I’ll be able to come back in a few weeks to see that memcached has saved me thousands of hits to the mysql database, therefore increasing the speed with which the Open Guide to Boston loads, something that I’m sure everyone can appreciate.

Most of the slowness that memcached is fixing in this case is not caused by MySQL being slow, but rather by the processing done by OpenGuides after the fact, due to the way that it is designed. The software was never really built to be the most efficient Wiki software out there, but to scale to the extent that it needed to, a task which it has completed admirably. However, this has left some corner cases where the performance was less than perfect — typically cases which were seldom used. The example of the “All nodes” map that the guide has is a good one: I built that page out of a corner case (the ?action=index view) which is not linked from the guide itself. As a result, I discovered a case where the software did not perform as expected. There was a roundtrip to the database to load the data about each node. This round trip time combined with a complex map { } function brought performance way below acceptable levels. So, I sought to improve that. First I simply cached all the data after it was processed, and today implemented caching of the individual nodes. Although the second change is minor and may not offer significant benefits on the small scale, it does help to build up the larger cache that it’s supporting: Roundtrips to memcached are quicker than roundtrips to the database.

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