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I have a website that has approximately 300 users and I get about 1000 hits per day. Due to an upcoming event next week, there is a possibility of all of a sudden getting between 20 and 25 thousand users per day, with between 60 and 70 thousand hits. I currently have 2 CPU on a semi-dedicated hosting. My host is aware that there could be a bump in users and said that he could quickly up the CPU as needed. However, there's also an option of prepaying for additional CPU. As the 20-25K users would be purchasing access to my site, I definitely want that aspect to go smoothly. Is there anyway to tell whether I should purchase access to the additional CPU?

Additional information:

1) A screen shot of my usage of the last week (the spikes are nightly database/file backups and won't increase with the new users)

2) I use a CDN for images, javascript, css

3) My "purchase access to the site" script takes about 2 seconds to run as it's interacting with my payment processor.

4) Each page that the user accesses requires a check via php to see if they're logged in, and 1-2 trips to the database: typically page loads are under 5 milliseconds.

If I'm being neurotic and shouldn't purchase more CPU, then that's fine for someone to tell me so. However, if this seems like a case where I should make the purchase, it would be great to know what number of additional CPU cores would be appropriate.

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Have you tried benchmarking or load testing the site to see how much traffic it can handle at the moment? Also, are you sure your current page loads are <5 ms? As that is very fast. –  Tim Fountain Mar 21 at 21:20
    
I'm a little bit embarrassed to say that I haven't done load testing; next week's event has caught me a bit off guard. And, when I say the "page loads", I'm referring to the time that php needs to execute: I save every page load time to my database and compute the mean page load nightly, so that I do feel confident about the 5ms figure. (I do lots of caching, use the CDN and my host uses LiteSpeed, which I think speeds things up significantly). –  Eric Mar 21 at 21:28
    
What are your hardware specs? How many other sites are on the server? It may be best to migrate to a dedicated server. I was a webhost in a prior life and I would have moved you to a dedicated server even if it was just for a while. I would have had a responsibility to you and the other sites to do the right thing. –  closetnoc Mar 21 at 23:40
    
There are 14 other sites sharing the server. In terms of hardware specs, it has 5GB disk space, 300GB bandwith, 2 CPU cores (my host said that I could purchase short term use 7-14 days of up to 6 more; if there were still issues, he said we would move to a dedicated server), 50 concurrent MYSQL connections. With all that said, if I did just request 2 or 3 more CPU core, would that be somewhat analogous to having a dedicated server? –  Eric Mar 22 at 0:21

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