On 12/7/22 3:32?PM, Gulam Mohamed wrote: > As per the review comment from Jens Axboe, I am re-sending this patch > against "for-6.2/block". > > > Use ktime to change the granularity of IO accounting in block layer from > milli-seconds to nano-seconds to get the proper latency values for the > devices whose latency is in micro-seconds. After changing the granularity > to nano-seconds the iostat command, which was showing incorrect values for > %util, is now showing correct values. > > We did not work on the patch to drop the logic for > STAT_PRECISE_TIMESTAMPS yet. Will do it if this patch is ok. > > The iostat command was run after starting the fio with following command > on an NVME disk. For the same fio command, the iostat %util was showing > ~100% for the disks whose latencies are in the range of microseconds. > With the kernel changes (granularity to nano-seconds), the %util was > showing correct values. Following are the details of the test and their > output: My default peak testing runs at 122M IOPS. That's also the peak IOPS of the devices combined, and with iostats disabled. If I enabled iostats, then the performance drops to 112M IOPS. It's no longer device limited, that's a drop of about 8.2%. Adding this patch, and with iostats enabled, performance is at 91M IOPS. That's a ~25% drop from no iostats, and a ~19% drop from the iostats we have now... Here's what I'd like to see changed: - Split the patch up. First change all the types from unsigned long to u64, that can be done while retaining jiffies. - Add an iostats == 2 setting, which enables this higher resolution mode. We'd still default to 1, lower granularity iostats enabled. I think that's cleaner than one big patch, and means that patch 1 should not really have any noticeable changes. That's generally how I like to get things split. With that, then I think there could be a way to get this included. -- Jens Axboe
