Charles Schwab, the largest discount and online brokerage, has started laying off employees as part of a plan announced in August to reduce the company's work force of about 22,000 by at least 9 percent. Dozens of workers at Schwab's headquarters in San Francisco were let go Monday, former employees said. A Schwab representative said the layoffs would continue for several weeks. The layoffs are the second round that Schwab has gone through this year. The economic slowdown has stunted the growth of the once explosive online brokerages, causing most to cut staff this year.