Hello, Paolo. On Thu, Aug 22, 2019 at 10:58:22AM +0200, Paolo Valente wrote: > Ok, I tried with the parameters reported for a SATA SSD: > > rpct=95.00 rlat=10000 wpct=95.00 wlat=20000 min=50.00 max=400.00 Sorry, I should have explained it with a lot more details. There are two things - the cost model and qos params. The default SSD cost model parameters are derived by averaging a number of mainstream SSD parameters. As a ballpark, this can be good enough because while the overall performance varied quite a bit from one ssd to another, the relative cost of different types of IOs wasn't drastically different. However, this means that the performance baseline can easily be way off from 100% depending on the specific device in use. In the above, you're specifying min/max which limits how far the controller is allowed to adjust the overall cost estimation. 50% and 400% are numbers which may make sense if the cost model parameter is expected to fall somewhere around 100% - ie. if the parameters are for that specific device. In your script, you're using default model params but limiting vrate range. It's likely that your device is significantly slower than what the default parameters are expecting. However, because min vrate is limited to 50%, it doesn't throttle below 50% of the estimated cost, so if the device is significantly slower than that, nothing gets controlled. > and with a simpler configuration [1]: one target doing random reads And without QoS latency targets, the controller is purely going by queue depth depletion which works fine for many usual workloads such as larger reads and writes but isn't likely to serve low-concurrency latency-sensitive IOs well. > and only four interferers doing sequential reads, with all the > processes (groups) having the same weight. > > But there seemed to be little or no control on I/O, because the target > got only 1.84 MB/s, against 1.15 MB/s without any control. > > So I tried with rlat=1000 and rlat=100. And this won't do anything as all rlat/wlat does is regulating how the overall vrate should be adjusted and it's being min'd at 50%. > Control did improve, with same results for both values of rlat. The > problem is that these results still seem rather bad, both in terms of > throughput guaranteed to the target and in terms of total throughput. > Here are results compared with BFQ (throughputs measured in MB/s): > > io.weight BFQ > target's throughput 3.415 6.224 > total throughput 159.14 321.375 So, what should have been configured is something like $ echo '8:0 enable=1 rpct=95 rlat=10000 wpct=95 wlat=20000' > /sys/fs/cgroup/io.cost.qos which just says "target 10ms p(95) read latency and 20ms p(95) write latency" without putting any restrictions on vrate range. With that, I got the following on Micron_1100_MTFDDAV256TBN which is a pretty old 256GB SATA drive. Aggregated throughput: min max avg std_dev conf99% 266.73 275.71 271.38 4.05144 45.7635 Interfered total throughput: min max avg std_dev 9.608 13.008 10.941 0.664938 During the run, iocost-monitor.py looked like the following. sda RUN per=40ms cur_per=2074.351:v1008.844 busy= +0 vrate= 59.85% params=ssd_dfl(CQ) active weight hweight% inflt% del_ms usages% InterfererGroup0 * 100/ 100 22.94/ 20.00 0.00 0*000 023:023:023 InterfererGroup1 * 100/ 100 22.94/ 20.00 0.00 0*000 023:023:023 InterfererGroup2 * 100/ 100 22.94/ 20.00 0.00 0*000 025:023:021 InterfererGroup3 * 100/ 100 22.94/ 20.00 0.00 0*000 023:023:023 interfered * 36/ 100 8.26/ 20.00 0.42 0*000 003:004:004 Note that interfered is reported to only use 3-4% of the disk capacity while configured to consume 20%. This is because with single concurrency 4k randread job, its ability to consume IO capacity is limited by the completion latency. 10ms is pretty generous (ie. more work-conserving) target for SSDs. Let's say we're willing to tighten it to trade off total work for tighter latency. $ echo '8:0 enable=1 rpct=95 rlat=2500 wpct=95 wlat=5000' > /sys/fs/cgroup/io.cost.qos Aggregated throughput: min max avg std_dev conf99% 147.06 172.18 154.608 11.783 133.096 Interfered total throughput: min max avg std_dev 17.992 19.32 18.698 0.313105 and the monitoring output sda RUN per=10ms cur_per=2927.152:v1556.138 busy= -2 vrate= 34.74% params=ssd_dfl(CQ) active weight hweight% inflt% del_ms usages% InterfererGroup0 * 100/ 100 20.00/ 20.00 386.11 0*000 070:020:020 InterfererGroup1 * 100/ 100 20.00/ 20.00 386.11 0*000 070:020:020 InterfererGroup2 * 100/ 100 20.00/ 20.00 386.11 0*000 070:020:020 InterfererGroup3 * 100/ 100 20.00/ 20.00 0.00 0*000 020:020:020 interfered * 100/ 100 20.00/ 20.00 1.21 0*000 010:014:017 The followings happened. * The vrate is now hovering way lower. The device is now doing less total work to acheive tighter completion latencies. * The overall throughput dropped but interfered's utilization is now significantly higher along with its bandwidth from lower completion latencies. For reference: [Disabled] Aggregated throughput: min max avg std_dev conf99% 493.98 511.37 502.808 9.52773 107.621 Interfered total throughput: min max avg std_dev 0.056 0.304 0.107 0.0691052 [Enabled, no QoS config] Aggregated throughput: min max avg std_dev conf99% 429.07 449.59 437.597 8.64952 97.7015 Interfered total throughput: min max avg std_dev 0.456 3.12 1.08 0.774318 Thanks. -- tejun
