Hi folks, I wanted to check log IO speed impact during fsync-heavy workload. To obtain theoretical maximum performance data, I did fake-completion of all log IOs (i.e. log IO cost is made 0). Test: iozone, single-thread, 1GiB file, 4K record, sync for each 4K ( '-eo' option). Disk: 800GB NVMe disk. XFS based on 4.15, default options except log size = 184M Machine: Intel Xeon E5-2690 @2.6 GHz, 2 NUMA nodes, 24 cpus each And results are : ------------------------------------------------ baseline log fake-completion 109,845 45,538 ------------------------------------------------ I wondered why fake-completion turned out to be ~50% slower! May I know if anyone encountered this before, or knows why this can happen? For fake-completion, I just tag all log IOs bufer-pointers (in xlog_sync). And later in xfs_buf_submit, I just complete those tagged log IOs without any real bio-formation (comment call to _xfs_bio_ioapply). Hope this is correct/enough to do nothing! It seems to me that CPU count/frequency is playing a role here. Above data was obtained with CPU frequency set to higher values. In order to keep running CPU at nearly constant high frequency, I tried things such as - performance governor, bios-based performance settings, explicit setting of cpu scaling max frequency etc. However, results did not differ much. Moreover frequency did not remain constant/high. But when I used "affine/bind" option of iozone (-P option), iozone runs on single cpu all the time, and I get to see expected result - ------------------------------------------------------------- baseline (affine) log fake-completion(affine) 125,253 163,367 ------------------------------------------------------------- Also, during above episode, I felt the need to discover best way to eliminate cpu frequency variations out of benchmarking. I'd be thankful knowing about it. Thanks, -- Joshi
