iotop is a tool to monitor I/O usage, it works with the Linux kernel starting at version 2.6.20. By default iotop works on a per-thread basis and shows information for each thread individually. This tool requires root access.

For the use case of analyzing a single program iotop offers the option to limit diagnostics to a single process. Replace PID with your process-id.

iotop -p PID

While running in interactive mode you can toggle between showing all threads and only those doing I/O by pressing o.

For analyzing a single programs performance you might want to log the results instead of viewing them in interactive mode. Use the '–batch' or '-b' option and log the results to a file.

iotop -bp PID > iostats.log

By default the tool shows disk read- and write speed per thread as well as the percentage of time spent waiting for swap and I/O. The I/O priority is also displayed.

Below you see part of the output of iotop while running cat /dev/urandom > random.file.

Total DISK READ :0.00 B/s | Total DISK WRITE :13.45 M/s
Actual DISK READ:0.00 B/s | Actual DISK WRITE:0.00 B/s
TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND
4879 be/4  someuser 0.00 B/s   13.45 M/s   0.00 %     0.00 % cat /dev/urandom
1    be/4  root     0.00 B/s   0.00 B/s    0.00 %     0.00 % init
2    be/4  root     0.00 B/s   0.00 B/s    0.00 %     0.00 % [kthreadd]