There are those who think that the world of new music is a fiercely competitive arena, with rivalries and backbiting and too many artists competing for too few opportunities. [...] perhaps for many people that’s an accurate picture. The prolific and influential composer-performer turns 80 on Wednesday, a milestone that will be celebrated this weekend with a three-concert series by his longtime collaborators, the Kronos Quartet. [...] in addition to his artistic legacy — a long and varied creative record that includes some of the most notable works in the history of minimalism and post-minimalism — Riley must hold some kind of record as the happiest and least stress-afflicted musician now working. After an hour’s worth of conversation during one of his visits to San Francisco recently, I could practically feel my blood pressure lowering. Some of that may be the effect of a bucolic lifestyle. Since 1974, he and his wife have lived on a ranch in Grass Valley, in the Sierra foothills in Nevada County, where he devotes himself to a combination of musical and agrarian pursuits. “One reason I like gardening,” he said, is that I get a lot of ideas when I’m watering plants or pulling weeds. [...] yes, Riley is an old-school hippie — he embraces the term with his usual amused contentment — and much of the sunny openness implied by that term shows up in his music, which has changed and evolved with winning fluency over the decades. [...] Young, whom he met at UC Berkeley in the ’50s, drew him into the world of experimentalism — a chapter that led to the 1964 creation of “In C,” the masterpiece of structured freedom that remains his best-known work. Throughout the late 1960s and ’70s, Riley immersed himself in musical improvisation, producing such groundbreaking albums as “Rainbow in Curved Air” but not writing anything down. “I was teaching at Mills College,” he recalled, and they sent in an audition tape which was just fantastic. [...] I got to San Francisco and there were people like William Corbett-Jones who could play circles around me. [...] I didn’t have the nerves of steel you need for classical music. Instead of the Conservatory, Riley wound up at San Francisco State University, where his classmates included such prominent experimentalists as Pauline Oliveros and Loren Rush. After a brief sojourn in Europe, he returned to the Bay Area, and at the San Francisco Tape Music Center unveiled “In C.”