What is the best webserver hardware configuration?

When I deploy a large server for web traffic, I need to ensure that HTTP requests are serviced with a minimum response time and many companies impose a Service Level Agreement (SLA) requiring that 90% of web page requests be served up with sub-second response time. 

It's considered rude to make your customers wait, especially on the web.  Today's web users have a low tolerance level for slow page loads and many will simple click the "back" button after waiting more than a second. 

Measuring web server response time

The most important factor to look at is your high water mark of HTTP requests per second and your end-to-end response time during this high-usage period.  This response time is the total elapsed time between receipt of the request and the end of sending the requested HTML.  Some use the "Time To First Byte"(TTBF) metric to measure webserver response time.  Also see Webserver Traffic Monitoring Tips.

For a real measure of end-to-end response time, you can use open  gateway proxy web servers and see how fast you pages load from anywhere in the world.  To use a proxy server, you can use the proxy feature in Internet Explorer or FireFox, enter the IP address of the proxy server and time your HTTP web page requests.

The best configuration for web servers

So, how do you know when you have an appropriate web server configuration?  When a web server without a web cache is experiencing stress, the cause is most likely disk enqueue requests.  The average disk latency is 15-20 milliseconds and it does not take much traffic to cause disk enqueues. 

• Disk placement counts - If you have all of your images and HTML on adjacent cylinders you may experience latency from the movement of the read-write heads.  Disk I/O latency goes up, and the disk will shake like an out-of-balance washing machine as competing HTTP requests try to get their images and HTML.  You can use the Linux iostat utility to monitor your disk latency.  If you don't have a web cache, consider using super-fast solid-state disks.  It's also important to use direct I/O on your web server.
   

• Web Servers are not CPU intensive - Servicing an HTTP request does not involve high CPU consumption.  You can monitor your web server CPU consumption with the Linux top and vmstat utilities, and you have CPU enqueues when the runqueue exceeds the number of processors on the webserver. However, you can utilize idle CPU resources by installing HTTP compression to improve webserver HTML delivery speed, trading off unused CPU cycles for faster web response time.
   

• RAM matters most - Most webservers have some pages that are far more popular than others, and caching the most popular images will dramatically improve total response time.  Without a web cache, RAM usage should be minimal, but you should monitor for real RAM page-in conditions using vmstat, where page-in's correlate with scan rate.  Remember, page-in's occur as a normal part of program loading, and you must separate normal page-in's from those caused by server swapping.

Caching and the 80-20 rule

Disk speed is measured in milliseconds (thousandths of a second) and RAM is measured in nanoseconds (billionths of a second).  In theory, RAM should be 10,000 times faster than disk, but even with RAM management latency overhead, caching frequently-used components can results in a 500x improvement in webserver response time. 

Almost all web site have an 80-20 rule where 80% of the traffic is for the 20% of the most popular web pages.  You can examine your referrer statistics to see the top 20% most popular pages and cache them in RAM.  Once sized and configured, your web cache should keep your most frequently requested images and HTML cached for outbound traffic.  This article on web page caching notes hat a RAM cache can make a huge impact on overall response time:

"Formulating a caching policy for a web site - determining which resources should not get cached, which resources should, and for how long - is a vital step in improving site performance."

You can have a front-end cached reverse proxy server to serve-up requests without forcing your server to go to disk.  Open source products such as Squid do a great job, but your web server must have dedicated RAM resources.

Choosing your web server hardware configuration

Depending on you tolerance for 404 timeouts and response time during peak usage times, you can choose webserver hardware to minimize disk latency (caching) while ensuring that you have enough CPU and RAM for the OS.  Most companies choose Intel-based web server hardware on a server with high-channel disks (to support high volume disk reads) and expandable RAM to cache the most frequently requested pages and images.

Make sure that you choose a server that has expandable RAM capacity, at least 4 gig with expansion to 16 gig, depending on the size of your working set of frequently-referenced pages and images.

If you have less than a million page views a month, you can buy powerful 64-bit PC's for webserver hardware, install Red Hat Linux advanced server with Squid and have a 1.5 gig web cache for super-fast web page delivery.  Dell also offers higher volume webservers at reasonable prices.

For larger webserver traffic volumes, many web hosting companies offer larger servers.  I see here there's a Dual Xeon EM64T 3.2GHz with 2GB RAM and two 200GB SATA drives in a RAID 1 configuration (their biggest machine), which is better than the box you're on now because of the RAIDed fast drives, more GHz, and 64 bit processors. All for only $275.00 a month! The only setback is that the max bandwidth on that is 1TB per month.

If you'd want even cheaper than that, I also see a Dual Xeon 2.8GHz EM64T with 2GB RAM and two 80GB SATA RAID-1 drives for only $210 a month.

For cheaper servers, we go non-RAID. There's a dual core Athlon X2 3800 with 1GB RAM and an 80GB hard drive for only $160 a month. You would be able to upgrade the RAM for only a little more, but at that point I go with the beefier server that has RAID on it for faster I/O.

 

BC has experts to assist you in choosing the best webserver hardware configuration for your mission-critical web site.