WIth @dafis's help, there's a version tagged cafe3 on the master branch which is better performing with ByteString. I also went ahead and interned ByteString as Int, converting the structure to IntMap everywhere. That's reflected on the new "intern" branch at tag cafe4. Still it can't do the full 35 days for all users. It comes close, however, to 30 days under ghc 6.12 with the IntMap -- just where 6.10 was with Map ByteString. Some profiling is in prof/ subdirectory, with the tag responsible and RTS profiling option in the file name; .prof are -P, and the rest are -hX. When I downsize the sample data to 1 million users, the whole run, with -P profiling, is done in 7.5 minutes. Something happens when tripling that amount. For instance, making -A10G may cause sefgault, after a fast run up to 10 days, then seeming stalling, and a dump of days up to 28 before the segfault. -A5G comes closest, to 30 days, when coupled with -H1G. It's not clear to me how to work -A and -H together. I'll work with Simon to investigate the runtime, but would welcome any ideas on further speeding up cafe4. -- Alexy