On Tue, Jun 1, 2010 at 9:04 AM, William A. Rowe Jr. <wr...@rowe-clan.net> wrote: [...] > Plus deflate may provide no benefit, and degrade performance, if the CPU > utilization is a greater concern than bandwidth utilization.
The CPU utilization is an interesting topic for me because I've been working on a related type of transform (minify of dynamic content) and I've gotten into the habit of measuring nanoseconds of processing time per byte of content. With that in mind, I just added in some instrumentation around the zlib calls in mod_deflate and found that the compression takes 30-60 ns/byte for buffers of a few KB. (This is with Apache 2.2.14 and zlib 1.2.3 on a 2.8GHz x64 processor.) To put that number in perspective, if you were to devote the equivalent of one CPU core worth of processing power on a web server host to compression, 30ns/byte means the host would be able to do real-time compression of ~266Mb/s of outbound traffic. Assume that your monthly bandwidth pricing is $10 per 95th percentile peak Mbps. Assume further that by turning on deflate you can reduce outbound bandwidth usage by 75% (i.e., you're getting 4:1 compression). Thus the CPU core that you've devoted completely to deflate processing will save you ~$2000 per month in bandwidth pricing. If the CPU core costs less than $24000 per year (amortized capital cost plus power, cooling, support, data center space, marginal cost of additional sysadmins needed to support each additional server, etc, etc), then you still come out ahead by turning on deflate. A few additional thoughts: 1. Speeding up the deflate implementation would be an unqualified win. Supposedly the recently-released zlib 1.2.5 is faster, but I don't have any data on it. 2. The best practice I've found for implementing compression in a web server is to do the compression in the server closest to the network edge. E.g., if you have a web server fronted by a proxy cache, do the compression dynamically in the proxy cache rather than the web server. That way the cache doesn't have to store multiple variants of each object. Similarly, if you're using a CDN for content that can benefit from gzip, ask your CDN if they can do conditional compression for non-problematic user-agents on-the-fly. -Brian
