Help with understanding Object and Database Cache

  • Resolved cloudantswvestjens

    (@cloudantswvestjens)


    4 years, 8 months ago

    I’d like some information regarding the Object and Database Caches.

    We are managing a WordPress installation for one of our customers, however once we enable W3TC and configure the Object and Database Caches (with Redis as backend, 3-server cluster setup) the performance does something funny: Instead of being lower than before, enabling W3TC + object/db cache increases the load by a factor of 4.

    How do these caches work under the hood? What do they do? Are there certain plugins or certain ways things can be programmed in which case they should not be used?

    CPU/memory/hardware should not be a limitation, it’s a 3-server setup running HAProxy, Varnish (currently not caching) and Apache MPM preforked with Redis as caching backend.

    Looking forward to your feedback.

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  • Plugin Contributor Marko Vasiljevic

    (@vmarko)

    Hello @cloudantswvestjens

    Thank you for the inquiry and I am happy to assist you with this.
    Just to confirm, you have enabled both Database and Object caching? Is Redis the caching method you are using for DB/Object caching?
    Also when you say “increases the load by a factor of 4.” can you please share those test results and your website URL?
    WordPress website utilizes database quite a lot. And database caching allows to decrease database usage. Database caching allows to cache database request results.
    Database caching can make your site faster if the time needed to get data from the database is more than the time needed to get data from the cache.
    Object Cache allows to get data from the database more efficiently. Its purpose is similar to what Database Cache does. But compared to Database Cache, Object cache is more efficient as it allows to process fewer requests.

    As a rule of thumb, Object Cache is not worth using on shared hosting for the same reason as Database Cache, i.e. database is often not the performance bottleneck, but disk input-output operations.
    Thanks!

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