What is Google’s crawl rate and How to change Google’s crawl rate?

Google has sophisticated algorithms that determine how much to crawl each site. It’s goal is to crawl as many pages from your site as Google can on each visit without overwhelming your server’s bandwidth.
You can change the crawl rate (the time used by Googlebot to crawl the site) for sites that are at the root level—for example, www.sitename.com and http://subdomain.sitename.com. You can’t change the crawl rate for sites that are not at the root level–for example, www.sitename.com/folder.
Note: Crawl rate refers to the speed of Googlebot’s requests during the crawl process. It doesn’t have any effect on how often Google crawl or how deeply Google crawl your URL structure.
To change the Google’s crawl rate:
i) On the Google’s Webmaster Tools Home page, click the site you want.
ii) Under Site Configuration, click on Settings.
iii) In the Crawl Rate Section, select the option you want.
The new Google’s crawl rate will be valid for 90 days.