mmap

Previous blog post on how to trace Firefox IO using bpftrace via official page_fault_user tracepoint left me a bit unsatisfied with how complicated it turned out. Complexity has potential to be error-prone and the syscall-tracing dependency makes it impossible to trace IO within the main executable. I decided to try reimplement the trace using my old approach of tracing ext4 functions that handle page-faults. This turned out to be much more robust....

Modern browsers are some of the most complicated programs ever written. For example, the main Firefox library on my system is over 130Mbytes. Doing 130MB of IO poorly can be quite a performance hit, even with SSDs! :). Few people seem to understand how memory-mapped IO works. There are no pre-canned tools to observe it on Linux, thus even even fewer know how to observe it. Years ago, when I was working on Firefox startup performance, I discovered that libraries were loaded backwards (blog1, blog2, paper, GCC bug) on Linux....