Re: [PATCH] net: add Documentation/networking/scaling.txt

On Mon, 2011-08-01 at 11:49 -0700, Rick Jones wrote:

On 07/31/2011 11:56 PM, Tom Herbert wrote:

quoted

Describes RSS, RPS, RFS, accelerated RFS, and XPS.

Signed-off-by: Tom Herbert<redacted>
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  Documentation/networking/scaling.txt |  346 ++++++++++++++++++++++++++++++++++
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diff --git a/Documentation/networking/scaling.txt b/Documentation/networking/scaling.txt
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+Scaling in the Linux Networking Stack
+
+
+Introduction
+============
+
+This document describes a set of complementary techniques in the Linux
+networking stack to increase parallelism and improve performance (in
+throughput, latency, CPU utilization, etc.) for multi-processor systems.

Why not just leave-out the parenthetical lest some picky pedant find a 
specific example where either of those three are not improved?

As I'm sure you're aware, there is often a trade-off between throughput
and latency.  It might be useful to provide some guidelines for
optimising each of the above.

[...]

quoted

+== RSS IRQ Configuration
+
+Each receive queue has a separate IRQ associated with it. The NIC
+triggers this to notify a CPU when new packets arrive on the given
+queue. The signaling path for PCIe devices uses message signaled
+interrupts (MSI-X), that can route each interrupt to a particular CPU.
+The active mapping of queues to IRQs can be determined from
+/proc/interrupts. By default, all IRQs are routed to CPU0.  Because a

Really?

The default affinity for most IRQs is all-CPUs.  At least on x86, that
really means CPU 0 only, so far as I can see.

[...]

quoted

+== Accelerated RFS Configuration
+
+Accelerated RFS is only available if the kernel is compiled with
+CONFIG_RFS_ACCEL and support is provided by the NIC device and driver.
+It also requires that ntuple filtering is enabled via ethtool.

Requires that ntuple filtering be enabled?

[...]

As a matter of fact, n-tuple filtering is enabled by default where
available.  So it might actually make more sense to say that RFS
acceleration can be *disabled* by disabling n-tuple filtering using
ethtool.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.