Hello everyone, I'm using Linux on a embedded system similar in spec to a desktop PC from 15 years ago (256 MB RAM, 800-MHz CPU, USB). The system's primary use is recording high-definition digital television programs. Typically, the storage sub-system consists of a recent hard-disk drive connected over USB (Hi-Speed, effective throughput ~20-30 MB/s), using a single ext4 partition (journal disabled), mounted noexec+noatime (trying to minimize metadata interference). Typical bit-rate for this HD content ~1-3 MB/s Data is accumulated in two 800-kB buffers; when one buffer is full, it is written to file (using write(2)), which was opened O_SYNC. (Note to self: try O_DSYNC instead of O_SYNC) If I plot the latency of the write(2) operation, 99% of them complete in under 80 ms. However, in rare cases, there is a huge latency spike (up to 800 ms). If several of these rare outliers occur in a row, the recording is messed up. I am trying to figure out the source of these latency spikes. It could be the OS, the USB controller, the HDD controller... I was hoping I could use ftrace to determine whether the problem came from the OS itself. Is that the best tool for the job? Any recommendations on how to proceed? Regards. [I would be grateful if you could CC me in your replies. Thanks!] -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
