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  1. #1
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    Resource Usage Problem on my website

    I am having a problem with my Wordpress site which appears to be using up to much CPU resources. This manifests itself in slow response times from the website and in cpanel I always have the message "CPU resources were limited for your site". I am using the W3 Total cache plugin to cache the site, which should (in theory) minimise CPU resources needed.

    I have attached the Resource Usage Details graph from Cpanel, where you can see that most of the time the resource usage is low, but it is constantly spiking up to max, which I think is causing the slow down.

    I don't know if I need to change to a different hosting company, or if there is some fundamental problem with the site, so any suggestions as to where to start looking into this would be appreciated.

    resource.jpg

  2. #2
    SitePoint Wizard silver trophy ldcdc's Avatar
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    What are you using? A VPS? Then a likely cause of trouble is lack of memory leading to swap usage. The hard disks being inherently slow, the CPU ends up waiting for them to deliver data.
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  3. #3
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    Thanks for the reply. I am using Shared Cloud Hosting (the Cloud Business package) from Webhosting UK (this is different to their VPS packages).

    How would I go about checking the RAM usage to see if that is causing a slow down as you suggest?

  4. #4
    SitePoint Wizard silver trophy ldcdc's Avatar
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    "CPU resources were limited for your site".
    This suggests that, even though this is shared hosting, they have a soft limit in place for each end user's CPU usage. Your site is may be too popular and no longer fitting the particular package you've bought. A cache plugin can help, but only to point. If you have many other plugins installed, it may be a good idea to think hard if you really need all of them. It is possible that one or more of these plugins is CPU heavy. Turn them off one by one and see in which case there's a significant improvement in performance.
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  5. #5
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    Thanks for the suggestions. I'm going to move the site to a different host first to see if that helps, then use the old version as a test site and try turning the plugins off as you have suggested.

  6. #6
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    try to turn off all your wordpress plugin
    see for a couple days, is there any change...?
    for wordpress users, i suggest you to look for web hosting that use litespeed or nginx as their web server
    both of these software faster than apache

  7. #7
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    Thanks everyone for the suggestions. I've made some big improvements to the site in terms of loading speed and I thought I would let you know what I did. The site has information about Nerja on the Costa del Sol in Spain, including holiday accommodation, which makes up the bulk of the pages and the bulk of the processor overhead. As such it has around 1000 pages, mainly created by custom PHP scripts from custom field data. The data for the rental properties is largely taken from the estate agents own databases, so the pages are created automatically (the script for this runs at night).

    1) Rather than just turn off plugins one by one, I installed another plugin called "Debug Queries" by Frank Bültge to analyse my db usage. I used this to identify which plugins were begin wasteful with resources and I then got rid of the worst offenders. For example Adrotate was making 28 database queries to display two banners - that was more than 50% of the queries on the page. I wrote my own much simpler version of Adrotate which has 0 database queries and does pretty much exactly the same thing.

    3) I tried various caching plugins but none of them were really flexible enough or effective enough to do what I wanted, so I decided to add my own caching code. The pages are created by custom PHP scripts, which build the pages from the database. Now the first time a page is rendered a copy is saved as a file on disk. Subsequent page loads come from the disk. This reduces the processor time by a factor of 10-20 (ie processor time is between 5-10% what it was previously). By writing the code myself I have the advantage of knowing exactly which parts of the page stay the same and which change, so only the static parts get saved to disk, plus I know when the cached versions need to be updated.

    Overall On a typical page I reduced the number of database queries from 50-100 to 5-10 and page loading time from 1-2+ seconds, to under 1 sec on average.

    I also tidied up the database in a few ways. For example Wordpress is quite wasteful in the way it uses the DB and keeps copies of all sorts of things in the DB that you don't really need. So I used a plugin called "Revision Removal" to remove page revisions from the database. I removed 4000 or so revisions in this way (I have 1000 or so posts in my blog) - database reduced in size from 1.5Mb to 1Mb. The comments are also quite wasteful of resources and I tidied these up as well (google for more info on this).

    Finally I moved to a different host, and I am now finding page delivery is much more consistent without the yawning pauses (sometimes 5-10 seconds) that sometimes occurred with the old host.

    If you want to have a look at the website this page http://mynerja.com/rentals/nerja-long-term-rentals/ is one of the heaviest resource users (at least it was before I wrote my caching code). It shows a list of all the rental properties on the website. If you look at the bottom of the page (just under where it says "Nerja Long Term Rentals") you will see it says "Took 0s" if you are seeing a cached version. That is the amount of time spent doing the database queries - without caching its about .3 -.4 of a second. At the very bottom of the page you will see the total number of queries and how long the entire page took to render. When I looked just now it showed "23 queries. 0.690 seconds." which is pretty fast. Before it routinely could take 2 or 3 seconds to load.

    One other thing, I also cached the header as I found the query to construct the navigation menu was taking far too long for something that is exactly the same (more or less) on every page. I used a str_replace on the cached version of the string returned by wp_list_pages() to get the current_page_item highlight back. In all caching the header saved me over 0.5 of a second on load time - and remember as it is the header, it is on every single page.

    Overall the work was well worth doing (even if it did take a few weeks ) and I hope this post gives some ideas to other thinking about optimising their wordpress installs.


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