block, bfq: stress that low_latency must be off to get max throughput (43c1b3d6) · Commits · 戴 / test

Documentation/block/bfq-iosched.txt

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	@@ -11,6 +11,13 @@ controllers), BFQ's main features are:
	groups (switching back to time distribution when needed to keep		groups (switching back to time distribution when needed to keep
	throughput high).		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		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		performing at most ~30K IOPS; at most ~50 KIOPS on faster CPUs. As a
	reference, 30-50 KIOPS correspond to very high bandwidths with		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,		real-time applications are privileged and experience a lower latency,
	as explained in more detail in the description of how BFQ works.		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		distribution. In fact, if it is enabled, then BFQ automatically
	increases the bandwidth share of privileged applications, as the main		increases the bandwidth share of privileged applications, as the main
	means to guarantee a lower latency to them.		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		timeout_sync
	------------		------------

block/bfq-iosched.c

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	@@ -56,6 +56,11 @@
	* rotational or flash-based devices, and to get the job done quickly		* rotational or flash-based devices, and to get the job done quickly
	* for applications consisting in many I/O-bound processes.		* 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		* 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		* 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		* in the latter paper full details on the main algorithm, as well as

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