The "crawl budget":
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There is also not a hard limit on our crawl. The best way to think about it is that the number of pages that we crawl is roughly proportional to your PageRank. So if you have a lot of incoming links on your root page, we'll definitely crawl that. Then your root page may link to other pages, and those will get PageRank and we'll crawl those as well. As you get deeper and deeper in your site, however, PageRank tends to decline.
Another way to think about it is that the low PageRank pages on your site are competing against a much larger pool of pages with the same or higher PageRank. There are a large number of pages on the web that have very little or close to zero PageRank. The pages that get linked to a lot tend to get discovered and crawled quite quickly. The lower PageRank pages are likely to be crawled not quite as often.
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The recent blogosphere babble re "page speed" I think is clarified by this:
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One thing that's interesting in terms of the notion of a crawl budget is that although there are no hard limits in the crawl itself, there is the concept of host load. The host load is essentially the maximum number of simultaneous connections that a particular web server can handle. Imagine you have a web server that can only have one bot at a time. This would only allow you to fetch one page at a time, and there would be a very, very low host load, whereas some sites like Facebook, or Twitter, might have a very high host load because they can take a lot of simultaneous connections.
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