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Diffstat (limited to 'Documentation/block/bfq-iosched.txt')
| -rw-r--r-- | Documentation/block/bfq-iosched.txt | 29 |
1 files changed, 22 insertions, 7 deletions
diff --git a/Documentation/block/bfq-iosched.txt b/Documentation/block/bfq-iosched.txt index 98a8dd5ee385..1a0f2ac02eb6 100644 --- a/Documentation/block/bfq-iosched.txt +++ b/Documentation/block/bfq-iosched.txt @@ -20,13 +20,26 @@ for that device, by setting low_latency to 0. See Section 3 for details on how to configure BFQ for the desired tradeoff between latency and throughput, or on how to maximize throughput. -BFQ has a non-null overhead, which limits the maximum IOPS that a CPU -can process for a device scheduled with BFQ. To give an idea of the -limits on slow or average CPUs, here are, first, the limits of BFQ for -three different CPUs, on, respectively, an average laptop, an old -desktop, and a cheap embedded system, in case full hierarchical -support is enabled (i.e., CONFIG_BFQ_GROUP_IOSCHED is set), but -CONFIG_DEBUG_BLK_CGROUP is not set (Section 4-2): +As every I/O scheduler, BFQ adds some overhead to per-I/O-request +processing. To give an idea of this overhead, the total, +single-lock-protected, per-request processing time of BFQ---i.e., the +sum of the execution times of the request insertion, dispatch and +completion hooks---is, e.g., 1.9 us on an Intel Core i7-2760QM@2.40GHz +(dated CPU for notebooks; time measured with simple code +instrumentation, and using the throughput-sync.sh script of the S +suite [1], in performance-profiling mode). To put this result into +context, the total, single-lock-protected, per-request execution time +of the lightest I/O scheduler available in blk-mq, mq-deadline, is 0.7 +us (mq-deadline is ~800 LOC, against ~10500 LOC for BFQ). + +Scheduling overhead further limits the maximum IOPS that a CPU can +process (already limited by the execution of the rest of the I/O +stack). To give an idea of the limits with BFQ, on slow or average +CPUs, here are, first, the limits of BFQ for three different CPUs, on, +respectively, an average laptop, an old desktop, and a cheap embedded +system, in case full hierarchical support is enabled (i.e., +CONFIG_BFQ_GROUP_IOSCHED is set), but CONFIG_DEBUG_BLK_CGROUP is not +set (Section 4-2): - Intel i7-4850HQ: 400 KIOPS - AMD A8-3850: 250 KIOPS - ARM CortexTM-A53 Octa-core: 80 KIOPS @@ -566,3 +579,5 @@ applications. Unset this tunable if you need/want to control weights. Slightly extended version: http://algogroup.unimore.it/people/paolo/disk_sched/bfq-v1-suite- results.pdf + +[3] https://github.com/Algodev-github/S |