path: root/Documentation
diff options
authorWillem de Bruijn <>2011-08-11 14:41:48 +0000
committerDavid S. Miller <>2011-08-13 18:00:33 -0700
commit320f24e482e6b390c608c6afec253405f9ab7436 (patch)
treecfb6f94a5bb381ab6794270a9db032d8c4a82c0b /Documentation
parentb88cf73d9278a5838e3ac2b670ab3b4ff533ea17 (diff)
net: minor update to Documentation/networking/scaling.txt
Incorporate last comments about hyperthreading, interrupt coalescing and the definition of cache domains into the network scaling document scaling.txt Signed-off-by: Willem de Bruijn <> Signed-off-by: David S. Miller <>
Diffstat (limited to 'Documentation')
1 files changed, 15 insertions, 8 deletions
diff --git a/Documentation/networking/scaling.txt b/Documentation/networking/scaling.txt
index 7254b4b5910e..58fd7414e6c0 100644
--- a/Documentation/networking/scaling.txt
+++ b/Documentation/networking/scaling.txt
@@ -52,7 +52,8 @@ module parameter for specifying the number of hardware queues to
configure. In the bnx2x driver, for instance, this parameter is called
num_queues. A typical RSS configuration would be to have one receive queue
for each CPU if the device supports enough queues, or otherwise at least
-one for each cache domain at a particular cache level (L1, L2, etc.).
+one for each memory domain, where a memory domain is a set of CPUs that
+share a particular memory level (L1, L2, NUMA node, etc.).
The indirection table of an RSS device, which resolves a queue by masked
hash, is usually programmed by the driver at initialization. The
@@ -82,11 +83,17 @@ RSS should be enabled when latency is a concern or whenever receive
interrupt processing forms a bottleneck. Spreading load between CPUs
decreases queue length. For low latency networking, the optimal setting
is to allocate as many queues as there are CPUs in the system (or the
-NIC maximum, if lower). Because the aggregate number of interrupts grows
-with each additional queue, the most efficient high-rate configuration
+NIC maximum, if lower). The most efficient high-rate configuration
is likely the one with the smallest number of receive queues where no
-CPU that processes receive interrupts reaches 100% utilization. Per-cpu
-load can be observed using the mpstat utility.
+receive queue overflows due to a saturated CPU, because in default
+mode with interrupt coalescing enabled, the aggregate number of
+interrupts (and thus work) grows with each additional queue.
+Per-cpu load can be observed using the mpstat utility, but note that on
+processors with hyperthreading (HT), each hyperthread is represented as
+a separate CPU. For interrupt handling, HT has shown no benefit in
+initial tests, so limit the number of queues to the number of CPU cores
+in the system.
RPS: Receive Packet Steering
@@ -145,7 +152,7 @@ the bitmap.
== Suggested Configuration
For a single queue device, a typical RPS configuration would be to set
-the rps_cpus to the CPUs in the same cache domain of the interrupting
+the rps_cpus to the CPUs in the same memory domain of the interrupting
CPU. If NUMA locality is not an issue, this could also be all CPUs in
the system. At high interrupt rate, it might be wise to exclude the
interrupting CPU from the map since that already performs much work.
@@ -154,7 +161,7 @@ For a multi-queue system, if RSS is configured so that a hardware
receive queue is mapped to each CPU, then RPS is probably redundant
and unnecessary. If there are fewer hardware queues than CPUs, then
RPS might be beneficial if the rps_cpus for each queue are the ones that
-share the same cache domain as the interrupting CPU for that queue.
+share the same memory domain as the interrupting CPU for that queue.
RFS: Receive Flow Steering
@@ -326,7 +333,7 @@ The queue chosen for transmitting a particular flow is saved in the
corresponding socket structure for the flow (e.g. a TCP connection).
This transmit queue is used for subsequent packets sent on the flow to
prevent out of order (ooo) packets. The choice also amortizes the cost
-of calling get_xps_queues() over all packets in the connection. To avoid
+of calling get_xps_queues() over all packets in the flow. To avoid
ooo packets, the queue for a flow can subsequently only be changed if
skb->ooo_okay is set for a packet in the flow. This flag indicates that
there are no outstanding packets in the flow, so the transmit queue can