diff options
| -rw-r--r-- | Documentation/block/bfq-iosched.txt | 17 | ||||
| -rw-r--r-- | block/bfq-iosched.c | 5 |
2 files changed, 21 insertions, 1 deletions
diff --git a/Documentation/block/bfq-iosched.txt b/Documentation/block/bfq-iosched.txt index 1b87df6cd476..05e2822a80b3 100644 --- a/Documentation/block/bfq-iosched.txt +++ b/Documentation/block/bfq-iosched.txt @@ -11,6 +11,13 @@ controllers), BFQ's main features are: groups (switching back to time distribution when needed to keep throughput high). +In its default configuration, BFQ privileges latency over +throughput. So, when needed for achieving a lower latency, BFQ builds +schedules that may lead to a lower throughput. If your main or only +goal, for a given device, is to achieve the maximum-possible +throughput at all times, then do switch off all low-latency heuristics +for that device, by setting low_latency to 0. Full details in Section 3. + On average CPUs, the current version of BFQ can handle devices performing at most ~30K IOPS; at most ~50 KIOPS on faster CPUs. As a reference, 30-50 KIOPS correspond to very high bandwidths with @@ -375,11 +382,19 @@ default, low latency mode is enabled. If enabled, interactive and soft real-time applications are privileged and experience a lower latency, as explained in more detail in the description of how BFQ works. -DO NOT enable this mode if you need full control on bandwidth +DISABLE this mode if you need full control on bandwidth distribution. In fact, if it is enabled, then BFQ automatically increases the bandwidth share of privileged applications, as the main means to guarantee a lower latency to them. +In addition, as already highlighted at the beginning of this document, +DISABLE this mode if your only goal is to achieve a high throughput. +In fact, privileging the I/O of some application over the rest may +entail a lower throughput. To achieve the highest-possible throughput +on a non-rotational device, setting slice_idle to 0 may be needed too +(at the cost of giving up any strong guarantee on fairness and low +latency). + timeout_sync ------------ diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index bd8499ef157c..08ce45096350 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -56,6 +56,11 @@ * rotational or flash-based devices, and to get the job done quickly * for applications consisting in many I/O-bound processes. * + * NOTE: if the main or only goal, with a given device, is to achieve + * the maximum-possible throughput at all times, then do switch off + * all low-latency heuristics for that device, by setting low_latency + * to 0. + * * BFQ is described in [1], where also a reference to the initial, more * theoretical paper on BFQ can be found. The interested reader can find * in the latter paper full details on the main algorithm, as well as |