I happened to find your “Optimizing MySQL on source code level” slides for mysql conference 2006 ( http://www.mysqlperformanceblog.com/files/presentations/UC20 06-Optimizing-MySQL-on-Source-Code-Level.pdf ). In your slides, you talked about key_root_lock rwlock that cause the bad scaling of MyISAM. And by disabling concurrent_insert feature, the scaling is improved.
I recently used standard sysbench (read only mode, 14 reads) to test MySQL5.1.22. I found that the thread scaling is ~1.52x from 2 threads to 4 threads (maybe acceptable for some guys) with concurrent_insert enabled. While with concurrent_insert disabled, the thread scaling improved to ~1.9x from 2 threads to 4 threads. Wow, this scaling 1.9x is what I want. The test environment is 2 x Intel XEON Dual-Core processors and SuSE10 (22.214.171.124-0.12-smp).
From my understanding, disabling concurrent_insert feature is only beneficial to read-intensive application. However for real read/write mix applications, I think still need to enable concurrent_insert feature. Do you know how much benefit concurrent_insert could give?