In Calls for Market Reform, Multiple Voices The call for market reform, once confined to the industry fringes, is growing louder. A number of events in Washington this week put the spotlight on high-frequency trading, the proliferation of new exchanges and other concerns about the structure of the market. At a Senate hearing on Thursday and a few industry conferences this week, executives at stock exchanges and trading firms laid out potential changes that could prevent disruptions like the technology glitch led to $440 billion of losses at the trading firm Knight Capital in early August. "If we had a blank canvas and we were able to redraw our market structure, it would never look like the model we use today," Chris Concannon, an executive at the high-speed trading firm Virtu Financial, said in front of the Senate subcommittee on securities, insurance and investment. The executives, in some cases, said the market had grown too complex and too fast to be safe. Maureen O'Hara, a professor at Cornell University, said at a conference Wednesday that technological advances had made it cheaper for ordinary investors to buy and sell stocks. But she noted that this progress created problems that industry participants had only started to recognize, like the increased the instability of the market. "It's easy for people to say the markets are great, but it's hard to quibble with the fact that no one wants to be there," Ms. O'Hara, who is the chairman of the board of the market technology firm ITG, said at the conference, which was held at Georgetown University. Such issues will be addressed at a roundtable hosted by the Securities and Exchange Commission on Oct. 2, where the market's recent problems will be discussed. Ahead of that, industry players are calling for a wide array of often conflicting changes to the market. Andy Brooks, the head of stock trading at T. Rowe Price, said at the Senate hearing that regulators should take immediate steps to slow down the market and eliminate rules that allow high-speed trading firms to operate. Larry Tabb, the head of the market research firm TabbGroup, disagreed with most of Mr. Brooks' ideas but gave six suggestions for changes that could make the markets operate more smoothly, including an effort to slow down the creation of new exchanges and other trading venues. "I think we have gone too far," Mr. Tabb said. We clearly have too many places to trade. Joe Mecane, the head of market structure issues at NYSE-Euronext, said at the Georgetown conference that the problems were too complex to be handled in a piecemeal fashion. He said that what was truly needed was a broader rethinking of the way the nation's markets were set up. "A more holistic review of market structure is warranted," Mr. Mecane said. Senator Jack Reed, a Rhode Island Democrat who called the hearing, said that the topic required more attention, potentially in further hearings. "We have recognized benefits from the increased liquidity but now we have to step back and see, at what cost, and how do we make improvements, not just keep pressing along," Mr. Reed said.